Extension version checking

[ahesham-arm]

This is a proposal to improve how we check for CL extensions.

The goal is to centralise how we check for extensions and the extension version. The change ensures that, unless otherwise explicitly stated by the test, the device is queried to check for the latest version (as specified in the CL headers) of an extension.

If we agree to this change I can then have follow-up work that uses the new macros everywhere, instead of manual checks.

The pull request is split into commits for easier reviewing, I suggest you review each commit on its own, instead of the pull request as a whole.

[bashbaug]

Removing "focused review" for now while discussions are ongoing.