Graduated force-triggering (prereqs, xtriggers, queues).

[hjoliver]

Close #5572 (and extends it to push ext-triggers as well)

• If a task has unsatisfied prerequisites, cylc trigger will satisfy them
• Otherwise, it will satisfy any unsatisfied xtriggers
• Otherwise, it will submit the task to run despite queue limiting

The runahead limit is ignored (i.e., you can trigger tasks beyond the limit).

Reviewers - this is an important part of required intervention capability. E.g. if starting a flow in the middle of a cycle point, we need to be able to set up clock-triggered tasks in subsequent cycles to start checking their clock-triggers now, but not to run immediately.

@dpmatthews - pinging you for a quick look, as the brains behind the xtrigger tweak on this branch.

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).
• CHANGES.md entry included if this is a change that can affect users
• Cylc-Doc](https://github.com/cylc/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]

For the record, the way users force-spawn an xtriggered task such that it starts checking its xtrigger now, as opposed to actually running now, really falls into the cylc set proposal discussion. But we need this particularly capability fairly urgently, so temporarily homing it here in cylc trigger would be better than delaying it for long. [Discussed more on #5572]

[hjoliver]

(Probably superseded by the newly agreed intervention plans,)

[oliver-sanders]

Converted to draft to remove this from the review list for now. There may well be bits of this we will want to salvage.

[hjoliver]

Superseded by #5658

(I'll close this once I get a chance to do a quick salvage investigation).

[oliver-sanders]

@hjoliver, ready to close this one?