feat!: add cause to blob release arguments

[alanshaw]

A Piri storage node currently has no way to verify the legitimacy of a /blob/release invocation beyond trusting the upload service that issued it. /blob/release is the upload service's translation of a client /blob/remove, so the fix is to make the release carry proof of that origin: a Cause field holding the CID of the /blob/remove task, with the remove invocation itself travelling as metadata in the request container. The node then verifies:

• the /blob/remove invocation identified by Cause is present in the request container,
• its subject equals the release's Space,
• its digest argument equals the release's Digest

[bajtos]

I find it a bit weird that we are only adding a field to ReleaseArguments struct, with no behaviour changes. How can we verify that this is the right thing? Is there any downstream pull request showing how this new field will be used in our services?

[alanshaw]

So, Sprue makes this invocation. There is currently no code or PR to update.

Here's the PR for Piri (targets @frrist's PR for adding removal functionality): fil-forge/piri#45