Add multiple strategies for querying GitHub release and assets info

[wagoodman]

The github v4 api is a little heavy for the github installer strategy in binny. Specifically a github token is required for use at all, and though the v3 api does not require this, it fetches a lot more information (is slower) and has pretty strict rate limiting. For this reason I've decided to add multiple strategies for fetching release information.

For installing, 3 strategies are attempted in the following order:

• scrape the https://github.com/<user>/<repo>/releases/expanded_assets/<tag-or-latest> endpoint that drives the github UI. This is brittle, however, is the quickest way to get a list of all assets for a release universally. The risk is that GitHub may decide to change this endpoint at any time in the future for no reason or warning.
• guess a variety of checksums.txt assets that may or may not be there, then use the contents for a listing of assets. This is not universally guaranteed and there are a few common patterns to try so it is not an ideal solution to use as a default.
• use the v4 api as done today, which requires a token. (note: a better error message is now provided when there is no token available).

For updating the version locks, 2 strategies are attempted in the following order:

• fetch the latest release tag from https://github.com/<user>/<repo>/releases/latest (accepting application/json)
• use the v4 api as done today to get all releases, which requires a token.

For an update strategy to be deemed working, the result must not be empty and must also satisfy any user-given version constraint, otherwise the next strategy is attempted.

These updates will make binny not only faster but also not require a github token by default 🎉