Enable retrieving Changelogs via XML file

[teleksterling]

When viewing Upgradeable Plugins in the QGIS Plugin Manager I think it would be sensible for it to display show the changelog of the version available for upgrading to. But currently that isn't available to QGIS (without downloading each plugin zip file).

The first step to adding it to the UI is to make the data accessible!

I'm going to assume that adding all the changelogs to plugins.xml is a bad idea, so my proposed solutions are:

1. Add a changelogs.xml containing only (say) these columns:
   plugin_id, name, version, changelog
2. Add a URI endpoint that would return all metadata for a single plugin (and have the UI request each plugin of interest).
   For example: r"^(? P<package_name>[A-Za-z][A-Za-z0-9-_]+)/version/(?P<version>[^\/]+)/changelog/$"

(A third option (returning a filtered set of changelogs) I also assume would be undesirable for caching & server load reasons.)

I'm willing to work on a PR, but I don't know if this is something that's been raised before, or whether it's not feasible for some other reason.

Thanks!
Rick