feat: add tag/attribute to add visibility of events send by the agent

[Litarnus]

This PR adds sentry.php.agent to tags/attributes so that its easier to see if the agent is being used or not within Sentry. The main goal of it is to give some sort of visibility into the agent and is a best effort solution. A better solution might follow in the future

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 27d2fc6. Configure here.}