Refactor base co-execution client out of Kubernetes specific stuff.

[jmchilton]

I want to implement a AWS Batch execution for Galaxy. The existing AWS Batch Runner (https://github.com/galaxyproject/galaxy/pull/13466):

• Would only work if Galaxy is also running in the same cloud and on the same mounts https://github.com/galaxyproject/galaxy/pull/13466/files#diff-4ae0bd7db779280f507dcc58709bcdceea7fcb3142cc529a18340d5755ce2af7R370
• Could otherwise benefit from the staging, dependency management, and code consistency of the Pulsar Kubernetes Co-Execution Approach.

In the reverse direction - adopting this approach to AWS Batch will provide further proof of the viability, encourage me to write documentation, and likely will prove easier to write remote tests for.

[jmchilton]

Documentation for this PR: https://pulsar.readthedocs.io/en/latest/containers.html. It includes TES execution.