Change GetFieldMapping API to broadcast requests to nodes hosting the relevant indices.

This is due to the fact that have to have mappers in order to return the response.

Closes #5177
[TEST] Added get field mapping test variant that tests with a cluster that has a master only node.
Added TransportGetFieldMappingsIndexAction that uses TransportSingleCustomOperationAction as base class, with the goal to reuse common logic (like: retry on failures, shard picking, connecting to nodes)