Add command for uploading Android mappings

[matthiaswenz]

Adds the crashes upload-mappings command with arguments

--mapping|-m -> for the mapping file name
--version-name|-n -> for the version name (marketing version i.e. "1.0")
--version-code|-c -> for the version code (build number, i.e. "1")

1. All three arguments are required. The version name and version code are required in App Center Diagnostics to associate the mappings with the right published app version
2. Android ProGuard mappings are directly uploaded as a mapping.txt file
3. Internally changed the upload of symbols to use the generated UploadSymbolsBeginRequest model and generified the upload symbols helper functionality to remove references to only uploading zip files as symbol artifacts

[bry-guy]

I left some comments which are largely advisory. I may take a deeper look at the tests still, but in concept LGTM :)

[owenniblock]

Just a minor note about the README.md text for future reference. Lovely tests. I've only cast my eye briefly over this as it already has 2 approves and is merged :)

[owenniblock]

This file is semi-automatically updated as part of the release process - you don't need to change it on the feature PRs :)