Add api methods for getting and updating user oauth2 applications.

Signed-off-by: Dan Molik <dan@danmolik.com>

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

* add API endpoint to create OAuth2 Application.

* move endpoint to /user. Add swagger documentations and proper response type.

* change json tags to snake_case. add CreateOAuth2ApplicationOptions to swagger docs.

* change response status to Created (201)

* add methods to list OAuth2 apps and delete an existing OAuth2 app by ID.

* add APIFormat convert method and file header

* fixed header

* hide secret on oauth2 application list

* add Created time to API response

* add API integration tests for create/list/delete OAuth2 applications.

Co-authored-by: techknowlogick <matti@mdranta.net>
Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: guillep2k <18600385+guillep2k@users.noreply.github.com>