APIv4 - Improve pseudoconstant support in getFields

[colemanw]

Overview

Makes APIv4 smarter about automatically loading option lists for getFields

Before

A custom getFields function always had to deal with loading its own options.

After

This allows APIv4 getFields arrays to specify a pseudoconstant key which is automatically transformed into an array of options in the right format.

[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).

[colemanw]

retest this please

[seamuslee001]

this seems fine and should be well covered by unit tests