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/oidc/types.go

@@ -173,10 +173,16 @@ func NewEncoder() *schema.Encoder {
 type Time int64
 
 func (ts Time) AsTime() time.Time {
+	if ts == 0 {
+		return time.Time{}
+	}
 	return time.Unix(int64(ts), 0)
 }
 
 func FromTime(tt time.Time) Time {
+	if tt.IsZero() {
+		return 0
+	}
 	return Time(tt.Unix())
 }