Support custom data on the Relationship Type form IF enabled via extension.

Support custom data on the Relationship Type form IF enabled via extension.

This is an effort at a way to genericise core forms that basically exist to do crud on an entity. We have a
number of fairly straight forward forms of this type in core and, in order to allow extensions to use
custom fields on a range of entities we should support editing them on these core crud forms.

To add custom data support to an entity we need to
a) add the custom data to the form using CRM_Custom_Form_CustomData::addToForm($this);
b) ensure that the entity is saved using an api call not a BAO call
c) add the custom data block to the tpl file - ie  {include file="CRM/common/customDataBlock.tpl"}

(the above is possible due to previous work to add support & simplify)

In this PR the adding of the customData is done in the EntityFormTrait, the api is previously converted
and the custom data is included by using a generic tpl to support metadata applied to the form.

By using metadata on the form we can also give extension writers a lot more control over what
is on the form as they can add to, alter, or remove the metadata in a php hook. This
is the crux of this issue https://github.com/civicrm/civicrm-core/pull/12078 & it likewise is
looking to use a generic field template to add fields based on metadata. A key difference between the 2 prs
is that one uses divs & the other a table & that might preclude close sharing of the approach.

Note this PR DOES have the impact of adding translate links to 2 localisable
relationship type fields that did not currently have them - I think this is a good thing?

Example of how to enable custom fields for RelationshipType in an extension.
```
      civicrm_api3('OptionValue', 'create', [
        'option_group_id' => 'cg_extend_objects',
        'name' => 'civicrm_relationship_type',
        'label' => ts('Relationship Type'),
        'value' => 'RelationshipType',
      ]);
```