Support userinfo endpoint for Generic OIDC SSO

[hosht]

What would you like to be added:

If userInfoEndpoint is provided by property, I'd like the application to fetch claims by making a request to the userinfo endpoint.
Is it possible to implement this feature?

Why is this needed:

Hi, PipeCD dev team.
Since the release v0.49.0, I have been trying to configure SSO using Generic OIDC.
Our team is using Okta as an identity provider.
I'm encountering an issue where Okta doesn't seem to be returning custom claims within the ID token.
The Documentation says the following:

The full set of claims for the requested scopes is available via the /oauth2/v1/userinfo endpoint. Call this endpoint using the access token.

https://developer.okta.com/docs/api/openapi/okta-oauth/guides/overview/#scope-dependent-claims-not-always-returned

it seems that the userInfoEndpoint property in SSOConfigOIDC isn't being used.(authorizationEndpoint and tokenEndpoint too)
https://pipecd.dev/docs-v0.49.x/user-guide/managing-controlplane/configuration-reference/#ssoconfigoidc

These are additional references from Okta.
https://support.okta.com/help/s/article/How-To-Add-Custom-Profile-Attributes-As-Claims-In-a-ID-Token-or-userinfo?language=en_US
https://developer.okta.com/docs/api/openapi/okta-oauth/oauth/tag/OrgAS/#tag/OrgAS/operation/userinfo

[t-kikuc]

@hosht
Thank you for trying the SSO by OIDC!

Please wait some weeks until supporting userInfoEndpoint. We're investigating the solution.
(authorizationEndpoint and tokenEndpoint will be later)

it seems that the userInfoEndpoint property in SSOConfigOIDC isn't being used.(authorizationEndpoint and tokenEndpoint too)

As you mentioned, userInfoEndpoint, authorizationEndpoint, and tokenEndpoint are not used...

Probably we need to fix around here:

pipecd/pkg/oauth/oidc/oidc.go

Lines 87 to 100 in b8070a9

	func (c *OAuthClient) GetUser(ctx context.Context, clientId string) (*model.User, error) {
	idTokenRAW, ok := c.Token.Extra("id_token").(string)
	if !ok {
	return nil, fmt.Errorf("no id_token in oauth2 token")
	}
	verifier := c.Provider.Verifier(&oidc.Config{ClientID: clientId})
	idToken, err := verifier.Verify(ctx, idTokenRAW)
	if err != nil {
	return nil, err
	}
	var claims jwt.MapClaims

[kumo-rn5s]

@t-kikuc let me take a look