Merge GetCommitmentProof with GetProof

[rach-id]

          The comment Rene referred to is about combining this with `GetProof`. The `GetProof` is originally intended to give blob commitment proof, instead of namespace proof, which is a bug. The namespace proof is already served by the Share module and we don't need to duplicate that.

I would do this change in this PR as we decided to integrate APIs Blobstream needs into the node nicely. I am open to doing that in a separate PR, though, if you think that's better, @rach-id, but as long we release these two PRs together.

Originally posted by @Wondertan in #3470 (comment)

[rach-id]

Might also be related to: #2289