Create `WorkflowRuntimeClientBase` with shared logic for ZMQ & SSH clients

[MetRonnie]

These changes close #4611

Might suggest a commit-by-commit review

Requirements check-list

• [x] I have read CONTRIBUTING.md and added my name as a Code Contributor.
• [x] Contains logically grouped changes (else tidy your branch by rebase).
• [x] Does not contain off-topic changes (use other PRs for other changes).
• [x] Applied any dependency changes to both setup.cfg and conda-environment.yml.
• [x] Does not need tests.
• [x] No change log entry required (invisible to users).
• [x] No documentation update required (what about https://cylc.github.io/cylc-doc/latest/html/reference/api/zmq.html ?)

[MetRonnie]

(Was trying to get the tests to run)

[MetRonnie]

(To be merged after #4274)

[oliver-sanders]

@dwsutherland could you take a look at this one especially from the ZMQ server side.