feat: Token Exchange (RFC 8693) (#255)

This change implements OAuth2 Token Exchange in OP according to RFC 8693 (and client code)

Some implementation details:

- OP parses and verifies subject/actor tokens natively if they were issued by OP
- Third-party tokens verification is also possible by implementing additional storage interface
- Token exchange can issue only OP's native tokens (id_token, access_token and refresh_token) with static issuer

example/server/storage/oidc.go

@@ -16,6 +16,9 @@ const (
 
 	// CustomClaim is an example for how to return custom claims with this library
 	CustomClaim = "custom_claim"
+
+	// CustomScopeImpersonatePrefix is an example scope prefix for passing user id to impersonate using token exchage
+	CustomScopeImpersonatePrefix = "custom_scope:impersonate:"
 )
 
 type AuthRequest struct {