Add JSON-RPC access to directories and server lists.

[riban-bw]

Short description of changes

Adds JSON-RPC notification of server lists from directories. Allows polling a directory for this info.

CHANGELOG: Added jamulusclient/pollServerList methods and jamulusclient/receivedServerList notification to JSON-RPC interface.

Context: Fixes an issue?

As discussed here, it may be advantageous to increase the control and monitoring provided by the JSON-RPC interface. This provides access to server lists on directories.

Does this change need documentation? What needs to be documented and how?

This PR adds the associated documentation of the new method and notification in the JSON-RPC md page.

Status of this Pull Request

What is missing until this pull request can be merged?

Checklist

• [X] I've verified that this Pull Request follows the general code principles
• [X] I tested my code and it does what I want
• [ ] My code follows the style guide
• [X] I waited some time after this Pull Request was opened and all GitHub checks completed without errors.
• [X] I've filled all the content above