Support create a self personal access token in UserApi

[loicgreffier]

Currently, the creation of personal access tokens for the authenticated user himself is not supported in the UserApi.

Unless I missed it, only the admin-level endpoint /users/:user_id/personal_access_tokens is supported.

It would be nice to add support for the /user/personal_access_tokens endpoint, which allows a user to generate a personal access token for themselves without requiring admin permissions.

[loicgreffier]

@jmini If this issue is proven, I can submit a PR.

[jmini]

I was checking PersonalAccessTokenApi but this seems to be something else.

A PR is welcomed for this addition.