Bug 31794: Add REST endpoint to get an authority

Bug 31794: Add REST endpoint to get an authority

To test:
1. Apply patch
2. Set RESTBasicAuth preference to true
3. Get the id of an authority
4. Make a GET request to /api/v1/authorities/{authid} with one of the following content type header
  - application/json
  - application/marcxml+xml
  - application/marc-in-json
  - application/marc
  - text/plain
5. Check that you get the authority in the corresponding format
6. Sign off

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Pedro Amorim <pedro.amorim@ptfs-europe.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>