Annotate Ledger in Github

[blueridger]

The ledger is not particularly human readable, which makes it harder to audit PRs and commits for grain distributions, identity merges, etc. Github has Annotation functionality, though, and it would be nice if a CI or a Github Action automatically added human-readable annotations to PRs / branches.

Here is an Action that creates annotations from JSON: https://github.com/marketplace/actions/annotations-action
JSON could be generated pretty easily by a new CLI in sourcecred/sourcecred. Also, sourcecred/sourcecred#2719 would make this even easier.