Select field: Use a REST Client 

As a developer, I want to use a REST Client to populate the options of a select fieldtype, so that I can enable linking to external content, and don't have to configure the same REST config in multiple places.

For example, listing the forms in an external CRM aka "External Forms".

Acceptance Criteria:

• A select fieldType can display a list of external content via a REST Client.
• Specify the REST Client to use via configuration and light development. No Java development is necessary.