oidc: add code challenge supported methods

Signed-off-by: hectorj2f <hectorf@vmware.com>

internal/oidc/discovery/discovery_handler.go

@@ -37,6 +37,7 @@ type Metadata struct {
 	TokenEndpointAuthMethodsSupported []string `json:"token_endpoint_auth_methods_supported"`
 	ScopesSupported                   []string `json:"scopes_supported"`
 	ClaimsSupported                   []string `json:"claims_supported"`
+	CodeChallengeMethodsSupported     []string `json:"code_challenge_methods_supported"`
 
 	// ^^^ Optional ^^^
 
@@ -64,6 +65,7 @@ func NewHandler(issuerURL string) http.Handler {
 		SubjectTypesSupported:             []string{"public"},
 		IDTokenSigningAlgValuesSupported:  []string{"ES256"},
 		TokenEndpointAuthMethodsSupported: []string{"client_secret_basic"},
+		CodeChallengeMethodsSupported:     []string{"S256"},
 		ScopesSupported:                   []string{"openid", "offline"},
 		ClaimsSupported:                   []string{"groups"},
 	}