Add api to check for missing indices

[eileenmcnaughton]

Overview

Adds an api to check for missing mysql indices. Currently there is an api to attempt to update / repair them but not just to check. This adds that.

Before

No api to get a list of missing indices

After

$missingIndices = civicrm_api3('System', 'getmissingindices', [])['values'];

Technical Details

I intend for follow up with fixes so a single table can be queried and potentially repaired

Comments

[civibot]

(Standard links)

• If this is your first pull-request for CiviCRM, please browse CONTRIBUTING.md for information about the development and testing processes.
• If you are reviewing this pull-request, you may wish to consult the test sites and the Review Standards (long template, short template).

[eileenmcnaughton]

See follow up #16146

[eileenmcnaughton]

test this please

[seamuslee001]

This looks fine to me, merge on pass

[colemanw]

Hmm, I think new apis should be added to v4 instead of v3.