Generate bb-browser compatible URLs for execution responses

Context

Buildbarn Browser (aka bb-browser) is a small web front-end for ContentAddressableStorage and ActionCache services. It especially display nice summary pages for Action messages including the build outcome, expected inputs and processed outputs as well as log snippets, which is very usefull for debugging a build.

The REAPI recently gain support for additional information in ExecuteResponse. Bazel Buildbarn is using that info. field to store a URL to the bb-browser page for the Action message in CAS, thus allowing the client who submitted that action to access aggregated information for its query.

Would be nice if BuildGrid could also generate that kind of URL. Ideally, it should only be a matter of giving the execution service a base URL in its configuration: BuildGrid would then start generating URLs of the form (bb-browser compatible) and attach them to ExecuteResponse messages:

${base_url}/action/${instance_name}/${action_digest_hash}/${action_digest_size_bytes}

Task Description

• Add CAS web front-end configuration for the execution controller.
• Generate URLs for Action messages and embed them in ExecuteResponse.

Acceptance Criteria

Any submitted build action are responded with a message containing a CAS web front-end link.