Export executed command time metrics

A remote execution service may return to the client a set of ExecutedActionMetadata that essentially are time metrics regarding the different phases of a build request execution. Some of these metrics are to be collected on worker-side:

• input_fetch_start_timestamp
• input_fetch_completed_timestamp
• execution_start_timestamp
• execution_completed_timestamp
• output_upload_start_timestamp
• output_upload_completed_timestamp

When BuildBox is used as a backend by a remote execution worker bot, it should allow exporting these information.