Release File assemble

This is experimental, it implements the release assemble api that is compatible with sentry-cli's releases file upload-sorcemaps. Necessary for glitchtip#13 (closed)

At this time, it's a strange mix of ported OSS Sentry code with simplifications. It's likely that we'll want to either simplify it further by committing to 1 file = 1 blob instead of chunking the blobs like sentry does. Or else fully support chunking blobs.