swarm/api: url scheme bzz-hash to get hashes of swarm content (#15238) (#15715)

* swarm/api: url scheme bzz-hash to get hashes of swarm content (#15238)

Update URI to support bzz-hash scheme and handle such HTTP requests by
responding with hash of the content as a text/plain response.

* swarm/api: return hash of the content for bzz-hash:// requests

* swarm/api: revert "return hash of the content for bzz-hash:// requests"

Return hashes of the content that would be returned by bzz-raw
request.

* swarm/api/http: handle error in TestBzzGetPath

* swarm/api: remove extra blank line in comment