AuthN: Move oauthserver to extsvcauth (#75972)

* AuthN: Move oauthserver to extsvcauth

* Codeowners

pkg/services/extsvcauth/oauthserver/oasimpl/session.go

@@ -0,0 +1,16 @@
+package oasimpl
+
+import (
+	"github.com/ory/fosite/handler/oauth2"
+	"github.com/ory/fosite/token/jwt"
+)
+
+func NewAuthSession() *oauth2.JWTSession {
+	sess := &oauth2.JWTSession{
+		JWTClaims: new(jwt.JWTClaims),
+		JWTHeader: new(jwt.Headers),
+	}
+	// Our tokens will follow the RFC9068
+	sess.JWTHeader.Add("typ", "at+jwt")
+	return sess
+}