Add CDN repo Source to allow retrieving specs from a web URL

[igor-makarov]

This PR complements #8280 in the main repo.

See all the details there.

[igor-makarov]

I've made (hopefully) all the requested changes.

[igor-makarov]

I've added a threadpool to handle all the concurrency, using ruby-concurrent.

In addition, I've moved all the optimizations that were previously in the analyzer, into the CDNSource file.

[amorde]

This is awesome! Would love to see tests for this before merging

[igor-makarov]

So I've picked all the nits, and made most of the implementation-related methods private.

I've begun adding unit tests. I'm using webrick for mocking the CDN server. I had to put its hookup in spec_helper to ensure no tests fail. I think it works better than VCR, because what I need is a read-only, static-hosted HTTP server that supports ETag out of the box.

[igor-makarov]

I've added the tests for CDNSource. It's now 93% covered, according to what I can see.

[igor-makarov]

I've pruned the new mock_cdn_repo_remote fixture to have significantly less files. Hope that's better.

[amorde]

Haven't gotten the chance to try it out yet but the diff looks good to me

[igor-makarov]

I've made the changes requested by @segiddins.