[WIP] Co-execution client for Google Cloud Platform Batch v1

[jmchilton]

Implements #390.

From https://pulsar.readthedocs.io/en/latest/containers.html#galaxy-and-shared-file-systems about the advantages of a Pulsar co-execution client (a container native paradigm) over implementing a Galaxy job runner (an interface designed for shared file systems and DRMAA-style batch of jobs, not containers):

The most glaring disadvantage of not using Pulsar in the above scenarios is that Galaxy must be deployed in the same container with the same mounts as the job execution environment. This prevents leveraging external cloud compute, multi-cloud compute, and makes it unsuitable for common Galaxy use cases such as large public instances, Galaxy’s leveraging institution non-cloud storage, etc… Even within the same cloud - a large shared file system can be an expensive prospect and Pulsar may allow making use of buckets and such more tractable.