feat: add LINE OIDC provider support

[DevRyuki]

What kind of change does this PR introduce?

Feature

This PR adds LINE(https://developers.line.biz/en/docs/line-login/integrate-line-login/) as an external provider.

What is the current behavior?

LINE provider did not exist before.

What is the new behavior?

Adds a first-class LINE OIDC provider, exposes configuration switches in settings, parses LINE ID tokens, and ensures tests cover the new provider so projects can enable LINE sign-in.

https://developers.line.biz/en/docs/line-login/integrate-line-login/

Additional context