uis subscription data sync info (UIS-597)

[dwsutherland]

sibling of https://github.com/cylc/cylc-uiserver/pull/597

Have included testing some of the changes, however the parts not tested are used by cylc-uiserver not cylc-flow.

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 if present).
• [x] Tests included (limited), as functionality is only used UIS side.
• [x] CHANGES.md entry included if this is a change that can affect users
• [x] If this is a bug fix, PR should be raised against the relevant ?.?.x branch.