Implement Platform controller and URLMappings in mod-agreements

Preliminary PR created to hopefully allow progress on the frontend
https://github.com/folio-org/mod-agreements/pull/378

Working:

• GET /erm/platforms/{$id}

• PUT /erm/platforms/{$id} - with postman, not successfully tested with curl yet

Partly working:

• GET /erm/platforms/ - works, but excludes any platform items that have been changed via database or endpoint

To be done:

• locators should not expand to second level (owner)

Current sample output for GET /erm/platforms:

view file also probably needed in `mod-agreements/service/grails-app/views`
Try something like

if that doesn't work there are a lot of examples of how to do specific things in other views, or reach out to me

The Module Descriptor will also need changing, `mod-agreements/service/src/main/okapi/ModuleDescriptor-template.json` to expose the endpoint and add permissions etc