feat: Allow groupIdentify distinct ID override

[dustinbyrne]

💡 Motivation and Context

groupIdentify() currently always uses a synthetic distinct ID like $team_team_x2, which can create one dummy person per group. Other server-side SDKs allow callers to override the distinct ID used for the $groupidentify event.

This adds a backwards-compatible optional distinctId / distinct_id override while preserving the current synthetic ID default when omitted.

💚 How did you test it?

• ./vendor/bin/phpcs --standard=phpcs.xml lib/PostHog.php test/RequestContextTest.php bin/posthog
• ./vendor/bin/phpunit --no-coverage --filter 'RequestContextTest'
• ./vendor/bin/phpunit --no-coverage (passes with existing warnings/deprecations)

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file