Retrieve GitHub release asset checksums from GitHub API

[jdx]

Discussed in #5464

^{Originally posted by risu729 June 29, 2025}
https://github.blog/changelog/2025-06-03-releases-now-expose-digests-for-release-assets/
On June 6th, GitHub started to generate SHA256 checksums for release assets.
We could use them to verify checksums for security.

Aqua backend currently verifies checksums if they are published and declared in the aqua-registry.
aqua has a proposal about this, but we still need to reimplement it to support.
aquaproj/aqua#3911

ubi doesn't support checksum validation, but it's tracked in houseabsolute/ubi#84.