Add GitHub Apps authentication as an alternative to personal access tokens

[AquaroTorres]

This PR adds support for authenticating with a GitHub App installation as an alternative to a personal access token (PAT). Many teams prefer GitHub Apps over PATs because they're scoped to specific repos/permissions, aren't tied to an individual user's account, and their tokens auto-expire — this brings that option to the package without changing
anything for existing PAT users.

What's new

• github_app config block, used instead of token:

'github_app' => [
    'client_id' => env('GITHUB_MONOLOG_APP_CLIENT_ID'),
    'installation_id' => env('GITHUB_MONOLOG_APP_INSTALLATION_ID'),
    'private_key' => env('GITHUB_MONOLOG_APP_PRIVATE_KEY'),
    // or: 'private_key_path' => env('GITHUB_MONOLOG_APP_PRIVATE_KEY_PATH'),
],

• GithubAppTokenProvider: signs a short-lived RS256 JWT for the app (using PHP's built-in openssl_sign; ext-openssl is a required PHP extension, and no third-party Composer package was added) and exchanges it for an installation access token via POST /app/installations/{id}/access_tokens.
• Token caching: installation tokens are valid for 60 minutes on GitHub's side; this caches them for 50 minutes (using the same cache store/prefix as the existing deduplication feature) to avoid re-authenticating on every request while staying safely under the real expiry.
• TokenProviderInterface abstraction: GithubIssueHandlerFactory now resolves either a PersonalAccessTokenProvider (existing PAT behavior, unchanged) or a GithubAppTokenProvider depending on config — github_app takes precedence if both are set.
• Private key flexibility: accepts the PEM contents directly, base64-encoded PEM (handy for single-line .env values), or a file path via private_key_path.
• README updates: setup walkthrough for creating the App, installing it, and finding the installation ID (including the organization-owned case), plus a note that GitHub's Client ID (not just the numeric App ID) works for the JWT iss claim, matching GitHub's current UI.

Backward compatibility
Fully opt-in — token continues to work exactly as before for anyone not using github_app. No breaking changes to the public API or existing config.

Testing

• New tests for GithubAppTokenProvider (JWT signing/verification, token caching, error handling for invalid keys and malformed API responses).
• Extended GithubIssueHandlerFactoryTest for the github_app resolution path, precedence over token, and the private_key_path fallback.
• Full suite passes (358 tests, 1096 assertions), pint clean.
• Long-lived workers: Handler refreshes cached GitHub App credentials once per logical write, allowing expired installation tokens to be replaced between writes.