Create username scope, required for clients to get username in ID token

- For backwards compatibility with older Pinniped CLIs, the pinniped-cli
  client does not need to request the username or groups scopes for them
  to be granted. For dynamic clients, the usual OAuth2 rules apply:
  the client must be allowed to request the scopes according to its
  configuration, and the client must actually request the scopes in the
  authorization request.
- If the username scope was not granted, then there will be no username
  in the ID token, and the cluster-scoped token exchange will fail since
  there would be no username in the resulting cluster-scoped ID token.
- The OIDC well-known discovery endpoint lists the username and groups
  scopes in the scopes_supported list, and lists the username and groups
  claims in the claims_supported list.
- Add username and groups scopes to the default list of scopes
  put into kubeconfig files by "pinniped get kubeconfig" CLI command,
  and the default list of scopes used by "pinniped login oidc" when
  no list of scopes is specified in the kubeconfig file
- The warning header about group memberships changing during upstream
  refresh will only be sent to the pinniped-cli client, since it is
  only intended for kubectl and it could leak the username to the
  client (which may not have the username scope granted) through the
  warning message text.
- Add the user's username to the session storage as a new field, so that
  during upstream refresh we can compare the original username from the
  initial authorization to the refreshed username, even in the case when
  the username scope was not granted (and therefore the username is not
  stored in the ID token claims of the session storage)
- Bump the Supervisor session storage format version from 2 to 3
  due to the username field being added to the session struct
- Extract commonly used string constants related to OIDC flows to api
  package.
- Change some import names to make them consistent:
  - Always import github.com/coreos/go-oidc/v3/oidc as "coreosoidc"
  - Always import go.pinniped.dev/generated/latest/apis/supervisor/oidc
    as "oidcapi"
  - Always import go.pinniped.dev/internal/oidc as "oidc"

site/content/docs/reference/code-walkthrough.md

@@ -188,7 +188,7 @@ The Supervisor's endpoints are:
 - And a number of endpoints for each FederationDomain that is configured by the user.
 
 Each FederationDomain's endpoints are mounted under the path of the FederationDomain's `spec.issuer`,
-if the issuer as a path specified in its URL. If the issuer has no path, then they are mounted under `/`.
+if the `spec.issuer` URL has a path component specified. If the issuer has no path, then they are mounted under `/`.
 These per-FederationDomain endpoint are all mounted by the code in
 [internal/oidc/provider/manager/manager.go](https://github.com/vmware-tanzu/pinniped/blob/main/internal/oidc/provider/manager/manager.go).
 
@@ -202,6 +202,10 @@ The per-FederationDomain endpoints are:
   See [internal/oidc/auth/auth_handler.go](https://github.com/vmware-tanzu/pinniped/blob/main/internal/oidc/auth/auth_handler.go).
 - `<issuer_path>/oauth2/token` is the standard OIDC token endpoint.
   See [internal/oidc/token/token_handler.go](https://github.com/vmware-tanzu/pinniped/blob/main/internal/oidc/token/token_handler.go).
+  The token endpoint can handle the standard OIDC `authorization_code` and `refresh_token` grant types, and has also been
+  extended in [internal/oidc/token_exchange.go](https://github.com/vmware-tanzu/pinniped/blob/main/internal/oidc/token_exchange.go)
+  to handle an additional grant type for [RFC 8693](https://datatracker.ietf.org/doc/html/rfc8693) token exchanges to
+  reduce the applicable scope (technically, the `aud` claim) of ID tokens.
 - `<issuer_path>/callback` is a special endpoint that is used as the redirect URL when performing an OIDC authcode flow against an upstream OIDC identity provider as configured by an OIDCIdentityProvider custom resource.
   See [internal/oidc/callback/callback_handler.go](https://github.com/vmware-tanzu/pinniped/blob/main/internal/oidc/callback/callback_handler.go).
 - `<issuer_path>/v1alpha1/pinniped_identity_providers` is a custom discovery endpoint for clients to learn about available upstream identity providers.