Make Akka.Discovery less coupled with Akka.Management

[Arkatufus]

Is your feature request related to a problem? Please describe.

Right now, Akka.Discovery is tightly coupled with Akka.Management.

• Only one Discovery plugin can be loaded via HOCON
• Discovery plugin retrieves its configuration from a fixed HOCON path
• A plugin can only exist once, no parallelization is allowed.

Describe the solution you'd like

• Change the configuration flow, have Akka.Discovery push the HOCON configuration to the ServiceDiscovery implementation instead of having the HOCON configuration path fixed Fix Akka.Discovery ServiceDiscovery instantiation #7245
• Make sure that ServiceDiscovery instantiation code is backward compatible with old code Fix Akka.Discovery ServiceDiscovery instantiation #7245
• Add a new public API in Akka.Discovery to retrieve a ServiceDiscovery instance Not needed, LoadServiceDiscovery() also doubles as a Get method.