Mark all code that interface with Commons or MediaInfo as beta

[dpriskorn]

According to https://phabricator.wikimedia.org/T149410 there is no stable interface policy for SDC currently.
Stable interfaces exist for:

• MediaWiki
• Wikidata

But not for

• Wikibase REST API (but it's versioned)
• Structured data on commons (and it's not versioned either)

[dpriskorn]

Or perhaps we should move it out of main and into a beta branch to get a stable library that just works towards stable endpoints?