schema: add first parent descendants

[oliver-sanders]

Closes cylc/cylc-ui#2071
Required for cylc/cylc-ui#1810

(POC PR to unblock upstream work, I'm not assigned, feel free to take over)

Add first-parent descendants into the schema.

Will require a back-compat change in cylc-uiserver to avoid schema errors with older schedulers.

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[hjoliver]

(This PR seems to be stuck on top of the NFS cache one.)