convenience functions for writing `RepoData`

[pavelzw]

after reworking rattler_index in #1076, i think it would be nice to add some convenience functions to generate the different repodata formats, including:

• [x] repodata.json
• [ ] repodata.json.bz2
• [ ] repodata.json.zst
• [ ] ~repodata.jlap~
• [ ] sharded repodata

rattler-index (and conda-mirror, which i'm currently also working on right now) could then use these functions to generate all repodata variants that are supported by pixi/conda/mamba right now.

• [x] we also should support repodata patching #1129

[baszalmstra]

I dont think we need to support jlap. The CEP never passed and it has been superseded by sharded repodata.

[pavelzw]

i also don't see much value in repodata.json.bz2 so let's skip that one as well...

[pavelzw]

#1312 should solve this