Add .gitignore entries for sensitive files

[pete-builds]

The .gitignore is missing entries for private keys and credentials (*.pem, *.key, *.p12, *.pfx, credentials.json, service-account*.json). Added those along with OS-specific files (.DS_Store, Thumbs.db) to help prevent accidental commits of sensitive material.

The existing cert/* entry covers certificates stored in that directory, but keys and credentials elsewhere in the tree weren't covered.

[Zaczero]

I don't mind that but is there any reason for it aside defensive coding?

[pete-builds]

Mostly defensive, yeah. If someone clones the repo and has credential files locally (from a deploy key, SSL cert, etc.), these patterns keep them from getting committed by accident. No urgent reason beyond that.