[identity] add user ID to account ownership message

Summary:
add user ID to the account ownership message. we use this user ID instead of minting a new one for existing users if they're already registered on keyserver but haven't yet registered with the identity service.

also added a condition expression to make sure we don't overwrite any user data accidentally.

Depends on D9480

Test Plan:
successfully registered a user with my local identity service who was previously registered with my local keyserver.

called the add_user_to_users_table function twice with the same user ID to test the condition expression. second call failed as expected.

Reviewers: bartek, michal, jon

Reviewed By: bartek

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9482