Optionally allow OIDC password grant for CLI-based login experience

- Add `AllowPasswordGrant` boolean field to OIDCIdentityProvider's spec
- The oidc upstream watcher controller copies the value of
  `AllowPasswordGrant` into the configuration of the cached provider
- Add password grant to the UpstreamOIDCIdentityProviderI interface
  which is implemented by the cached provider instance for use in the
  authorization endpoint
- Enhance the IDP discovery endpoint to return the supported "flows"
  for each IDP ("cli_password" and/or "browser_authcode")
- Enhance `pinniped get kubeconfig` to help the user choose the desired
  flow for the selected IDP, and to write the flow into the resulting
  kubeconfg
- Enhance `pinniped login oidc` to have a flow flag to tell it which
  client-side flow it should use for auth (CLI-based or browser-based)
- In the Dex config, allow the resource owner password grant, which Dex
  implements to also return ID tokens, for use in integration tests
- Enhance the authorize endpoint to perform password grant when
  requested by the incoming headers. This commit does not include unit
  tests for the enhancements to the authorize endpoint, which will come
  in the next commit
- Extract some shared helpers from the callback endpoint to share the
  code with the authorize endpoint
- Add new integration tests

deploy/supervisor/idp.supervisor.pinniped.dev_oidcidentityproviders.yaml

@@ -59,11 +59,44 @@ spec:
                   additionalScopes:
                     description: AdditionalScopes are the scopes in addition to "openid"
                       that will be requested as part of the authorization request
-                      flow with an OIDC identity provider. By default only the "openid"
-                      scope will be requested.
+                      flow with an OIDC identity provider. In the case of a Resource
+                      Owner Password Credentials Grant flow, AdditionalScopes are
+                      the scopes in addition to "openid" that will be requested as
+                      part of the token request (see also the AllowPasswordGrant field).
+                      By default, only the "openid" scope will be requested.
                     items:
                       type: string
                     type: array
+                  allowPasswordGrant:
+                    description: AllowPasswordGrant, when true, will allow the use
+                      of OAuth 2.0's Resource Owner Password Credentials Grant (see
+                      https://datatracker.ietf.org/doc/html/rfc6749#section-4.3) to
+                      authenticate to the OIDC provider using a username and password
+                      without a web browser, in addition to the usual browser-based
+                      OIDC Authorization Code Flow. The Resource Owner Password Credentials
+                      Grant is not officially part of the OIDC specification, so it
+                      may not be supported by your OIDC provider. If your OIDC provider
+                      supports returning ID tokens from a Resource Owner Password
+                      Credentials Grant token request, then you can choose to set
+                      this field to true. This will allow end users to choose to present
+                      their username and password to the kubectl CLI (using the Pinniped
+                      plugin) to authenticate to the cluster, without using a web
+                      browser to log in as is customary in OIDC Authorization Code
+                      Flow. This may be convenient for users, especially for identities
+                      from your OIDC provider which are not intended to represent
+                      a human actor, such as service accounts performing actions in
+                      a CI/CD environment. Even if your OIDC provider supports it,
+                      you may wish to disable this behavior by setting this field
+                      to false when you prefer to only allow users of this OIDCIdentityProvider
+                      to log in via the browser-based OIDC Authorization Code Flow.
+                      Using the Resource Owner Password Credentials Grant means that
+                      the Pinniped CLI and Pinniped Supervisor will directly handle
+                      your end users' passwords (similar to LDAPIdentityProvider),
+                      and you will not be able to require multi-factor authentication
+                      or use the other web-based login features of your OIDC provider
+                      during Resource Owner Password Credentials Grant logins. AllowPasswordGrant
+                      defaults to false.
+                    type: boolean
                 type: object
               claims:
                 description: Claims provides the names of token claims that will be