[CDRIVER-5537] A Signed Release Archive + Augmented SBOM Publication (#1637)

* New Earthly targets for release artifacts

- +release-archive generates an archive of the repository that contains
  the augmented SBOM downloaded from Silk
- +sign-file can sign arbitrary files
- +signed-release generates release artifacts using +release-archive
  and +sign-file to create a release archive and detached signature.

* Add documentation on using Earthly and the release targets
* Document new release steps for a signed release