release: create releases and upload artifacts #1355

atgc release can list, view and download a release's artifacts, but can't create a release. On Tangled, an artifact is an org.tangled.repo.artifact record in the repo's account on its knot, pointing at a blob uploaded to the knot.

Add atgc release create <tag> <files>, or release upload:

• upload each file to the knot with uploadBlob, addressed to the repo DID;
• write one artifact record per file, against the tag's object id;
• support --dry-run and --json.

Follow the web app's artifact upload in Tangled's web/src/lib/api.