dotnetup: Consider refactoring of release info provider APIs

Currently, the release info provider can return the latest `ReleaseVersion` in a channel. However, to install that version, the `DotnetArchiveDownloader` has to call `ReleaseManifest.FindReleaseFile` to get the download URL and hash.

It might be better if the methods to find a release version returned an object that also included the download URL, hash, and any other information needed to download and install the release.

Relatedly, it looks like we have different classes each creating their own ReleaseManifest instance. This means we may be downloading the same releases.json information multiple times in an operation. We should probably try to share a single ReleaseManifest across all the different clases that need it.