Feature add scan mutation

[markgrahamdawson]

Closes #377

This was going to bee added as a rest endpoint but after discussion with @oliver-sanders it was decided it would be better to add another mutation to uiserver

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). - NA
• 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 pull request opened if required at cylc/cylc-doc/pull/XXXX.- NA
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch. - NA

[oliver-sanders]

LGTM

[codecov-commenter]

Codecov Report

Patch coverage: 61.53% and project coverage change: -0.08% ⚠️

Comparison is base (0f3205a) 75.39% compared to head (25ba516) 75.31%.
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #493      +/-   ##
==========================================
- Coverage   75.39%   75.31%   -0.08%     
==========================================
  Files          12       12              
  Lines        1410     1422      +12     
  Branches      234      235       +1     
==========================================
+ Hits         1063     1071       +8     
- Misses        297      301       +4     
  Partials       50       50              

Files Changed	Coverage Δ
cylc/uiserver/resolvers.py	54.77% <28.57%> (-0.55%)	⬇️
cylc/uiserver/schema.py	72.22% <100.00%> (+1.20%)	⬆️

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[oliver-sanders]

LGTM

[MetRonnie]

@oliver-sanders Is that an approval or are you still yet to test etc?

[hjoliver]

LGTM too 👍