feat: add api for admin login

server/graph/schema.graphqls

@@ -62,6 +62,16 @@ type Response {
 	message: String!
 }
 
+type AdminLoginResponse {
+	message: String!
+	access_token: String!
+	expires_at: Int64!
+}
+
+input AdminLoginInput {
+	admin_secret: String!
+}
+
 input SignUpInput {
 	email: String!
 	given_name: String
@@ -153,6 +163,7 @@ type Mutation {
 	# admin only apis
 	_delete_user(params: DeleteUserInput!): Response!
 	_update_user(params: UpdateUserInput!): User!
+	_admin_login(params: AdminLoginInput!): AdminLoginResponse
 }
 
 type Query {
@@ -162,4 +173,5 @@ type Query {
 	# admin only apis
 	_users: [User!]!
 	_verification_requests: [VerificationRequest!]!
+	_admin_session: AdminLoginResponse
 }