Add repodata config to channel

[mariusvniekerk]

This builds on some of the idea of #420

When fetching from certain channels it would be handy to disable some of the repodata methods that are known to be expensive / non-functional.

This is particularly true for artifactory where the HEAD request for the .bz2 repodata will eager compress the contents and not cache it. :(

Idea is that in downstream tools like pixi a user can define their channel and add some of these options in

channels = [
   { base_url = "https://somehost/channel", repodata_options = { bz2_enabled = False }}
]

[baszalmstra]

Im a bit on the fence if adding this to channel makes sense responsibility wise. It makes sense when fetching repodata but its used in many more places were it doesnt make sense.

Let me mull it over a little.

[baszalmstra]

Prefixing with _ would mean its unused. I assume you want to make it private to the module, if you dont add "pub" that should be fine.

[baszalmstra]

I have been thinking about this and I think we need to alter the design a little bit. I think we need to split this functionality into two: Channel and FetchChannel (or whatever, naming is hard). The Channel only contains information about a url. The FetchChannel struct contains a Channel and extra metata information needed for fetching the channel's repodata. You should be able to construct a FetchChannel directly from a Channel with default values. And fetch_repodata function should accept a FetchChannel.

In pixi we can then change some of the logic to work with FetchChannel instead of Channel.

[baszalmstra]

With the new Gateway API, it is now possible to set configuration per channel.

Im closing this, feel free to reopen if you think this PR still adds more value.