Add pseudoconstant for Relationship.relationship_type_id

[colemanw]

Overview

Updates the relationship schema to add a pseudoconstant to the relationship_type_id field.

Before

No pseudoconstant declared, but APIv3 would return an option list for the field (but in a nonstandard format).

After

Pseudoconstant declared. APIv3 and APIv4 return option list in standard format, unless legacy "is_form" param is set.

Technical Details

This adds basic pseudoconstant support for the field using name_a_b and label_a_b (it does not support other properties like name_b_a)

For legacy purposes it preserves the work done in #11853 to return a list like [1_a_b => Child of, 1_b_a => Parent of] if the "is_form" flag is set.
This was very nonstandard but is used to support in-place editing of the relationship type list on the relationship form.

[civibot]

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[seamuslee001]

@colemanw test fails relate

[eileenmcnaughton]

Test Result (6 failures / -25)
api_v3_RelationshipTest.testRelationshipCreateWithIncorrectData with data set "APIv4"
api_v3_RelationshipTest.testGetTypeOperators with data set "APIv4"
api_v3_RelationshipTest.testGetRelationshipByTypeDAO with data set "APIv3"
api_v3_RelationshipTest.testGetRelationshipByTypeArrayDAO with data set "APIv4"
api_v3_RelationshipTest.testGetRelationshipByTypeReciprocal
api.v4.Entity.ConformanceTest.testConformance with data set "Relationship"

[eileenmcnaughton]

We discussed this & code approach seems OK - I'm going to give it a r-run

[eileenmcnaughton]

OK - this seems to work the same - @colemanw explained that for apiv3 it will be possible to pass the name_a value for the relationship - whereas the v4 api uses either name_a or name_b

[colemanw]

@eileenmcnaughton corection - both api versions will only use name_a_b and label_a_b not the inverse. But only api3 still accepts the oddball is_form for the sake of #11853