feat: merge the verifier types (#336)

BREAKING CHANGE:

- The various verifier types are merged into a oidc.Verifir.
- oidc.Verfier became a struct with exported fields

* use type aliases for oidc.Verifier

this binds the correct contstructor to each verifier usecase.

* fix: handle the zero cases for oidc.Time

* add unit tests to oidc verifier

* fix: correct returned field for JWTTokenRequest

JWTTokenRequest.GetIssuedAt() was returning the ExpiresAt field.
This change corrects that by returning IssuedAt instead.

pkg/op/token_request.go

@@ -20,8 +20,8 @@ type Exchanger interface {
 	GrantTypeJWTAuthorizationSupported() bool
 	GrantTypeClientCredentialsSupported() bool
 	GrantTypeDeviceCodeSupported() bool
-	AccessTokenVerifier(context.Context) AccessTokenVerifier
-	IDTokenHintVerifier(context.Context) IDTokenHintVerifier
+	AccessTokenVerifier(context.Context) *AccessTokenVerifier
+	IDTokenHintVerifier(context.Context) *IDTokenHintVerifier
 }
 
 func tokenHandler(exchanger Exchanger) func(w http.ResponseWriter, r *http.Request) {