Add custom response_mode=form_post HTML template.

This is a new pacakge internal/oidc/provider/formposthtml containing a number of static files embedded using the relatively recent Go "//go:embed" functionality introduced in Go 1.16 (https://blog.golang.org/go1.16).

The Javascript and CSS files are minifiied and injected to make a single self-contained HTML response. There is a special Content-Security-Policy helper to calculate hash-based script-src and style-src rules.

This new code is covered by a new integration test that exercises the JS/HTML functionality in a real browser outside of the rest of the Supervisor.

Signed-off-by: Matt Moyer <moyerm@vmware.com>

internal/oidc/callback/callback_handler_test.go

@@ -149,7 +149,7 @@ func TestCallbackEndpoint(t *testing.T) {
 			csrfCookie:                        happyCSRFCookie,
 			wantStatus:                        http.StatusOK,
 			wantContentType:                   "text/html;charset=UTF-8",
-			wantBodyFormResponseRegexp:        `<input type="hidden" name="code" value="(.+)"/>`,
+			wantBodyFormResponseRegexp:        `<code id="manual-auth-code">(.+)</code>`,
 			wantDownstreamIDTokenSubject:      upstreamIssuer + "?sub=" + queryEscapedUpstreamSubject,
 			wantDownstreamIDTokenUsername:     upstreamUsername,
 			wantDownstreamIDTokenGroups:       upstreamGroupMembership,