add configurable OAuth user field mappings

[Kreal321]

Fixes #43

Different OAuth providers use different field names in their userinfo endpoints. For example, Clerk uses user_id instead of id, which results in a null value for the required field. The solution is to add these configurations as new input fields in the admin page.

Changes proposed in this pull request:

• Add three new admin settings to customize OAuth response field names
• Support custom field mappings for user ID, email, and name fields
• Include database migration to initialize default field mappings
• Update documentation with field mapping examples and usage

Reviewers should focus on:

None. It’s a fairly simple change. I used the existing ResourceOwner::setFields design, which had been created earlier for extensions but wasn’t in use, minimizing any impact on the logic. Only three additional fields were added to the admin settings.

Screenshot

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).

Required changes:

None