Potentially use api_versions property for our version check

[bocops]

As described in this Mastodon blog post, the upcoming Mastodon v4.3.0 will add an api_versions property to the Instance entity. This is implemented as a hash consisting of potentially many String/Int pairs - although as a library for the Mastodon API, we will probably only need to access the mastodon key currently defined in the documentation.

• add property to our Instance entity
• discuss if we want/need to use this internally instead of some other form of API version check we currently employ

[bocops]

Mastodon's idea with this property seems to be to have an integer value at api_versions.mastodon that increases whenever the API changes, independent of the general server versioning. The nightly build currently running on mastodon.social reports a value of 2 (so the v4.3.0 release will probably report the same) - and as implemented in #484, it will default to 0 for all previous Mastodon versions.

We've discussed different ways of making our library safe to use against different server versions in the past, and this value might be helpful in that regard.