Add changeset endpoint

[leplatrem]

In kinto-changes we added a new .../changeset endpoint.

@glasserc do you think I could add it here? Or I should improve #742 to support that use-case?

Thanks for your feedback :)

[github-actions]

Size Change: +138 B (0%)

Total Size: 38.5 kB

Filename	Size	Change
dist/kinto-http.min.js	8.62 kB	+50 B (0%)
dist/kinto-http.node.js	14.3 kB	+53 B (0%)
dist/moz-kinto-http-client.js	15.5 kB	+35 B (0%)

_{compressed-size-action}

[glasserc]

I definitely think #742 is a nicer, more extensible approach to this problem. The server is built around the concept of plugins, so I guess the client should be too, right?

[leplatrem]

With what I propose in #742, it could become:

const path = client.endpoints.changeset(bid, cid);
await client.execute(path, { query: { _expected: 42 } });

I think it's better :) kinto-changes could be renamed kinto-remotesettings and I don't think this should go in the basic kinto http client