Add secret management to op.#FetchGit operation to fetch private repository

- Update `op.cue` to support secrets
- Update `pipeline.go` to use authTokenSecret & authHeaderSecret

Signed-off-by: Tom Chauveau <tom.chauveau@epitech.eu>

stdlib/dagger/op/op.cue

@@ -87,8 +87,8 @@ package op
 	ref:         string
 	keepGitDir?: bool
 	// FIXME: the two options are currently ignored until we support buildkit secrets
-	authTokenSecret?:  string | bytes
-	authHeaderSecret?: string | bytes
+	authTokenSecret?:  _ @dagger(secret)
+	authHeaderSecret?: _ @dagger(secret)
 }
 
 #FetchHTTP: {