Allow for custom_fields to be mapped to entities outside of backstage

[jasmeetsinghbhatia]

In my implementation of backstage the namespace is called default
And I have a custom_field called service that is mapped to catalog from another source (not backstage).
Which essentially means when i try to run the plugin against my incident.io api

the call translates to

http://localhost:7000/api/proxy/incident/api/v2/incidents?custom_field[field ID here][one_of]=[default/service-name]

This is causing problems because the custom field is pulling the names of services without any knowledge of what backstage metadata namespace is (in this case default)

So all the plugin queries return no incidents since the custom-field assigned to incidents is <service-name> and not default/service-name

This PR allows for option to include one of the 2 available field values with and without backstage namespace.