support splunk.discovery mapping in properties.discovery.yaml

[rmfitzpatrick]

These changes introduce a new mapped form for discovery.properties.yaml content:

splunk.discovery:
  extensions:
    observer_type:
      enabled: false
  receivers:
    receiver_type/receiver_name:
      enabled: false
      config:
        field: value

The properties in this form are compatible with the existing --set style ones in the same file* and have a lower sourcing precedence to account for collisions. Invalid properties loaded from the file will be ignored and logged as warnings so as not to interfere with collector startup.

In order to reuse an existing mergeMaps util it was migrated to a new internal package.

[theletterf]

I like this. Seems simpler!