Implemented an interface to support iterating over the options of a configuration section. (Bug #23337654)

The logging_folder configuration option now defaults to the MySQL Router base path instead of "". This also means that logs are no longer sent to stdout by default.

A get_options() function was added to iterate over all options in a configuration section. (Bug #23337654, Bug #81569)

Fixed CMake compilation warnings that were generated when building Router with tests enabled. (Bug #22276210)

Introduced support for Unix domain sockets for connections.

This adds a new optional configuration option named socket that can be used with connection routing. The name socket was borrowed from MySQL Server.

For example:

[routing:dev]
socket = /tmp/mysqlrouter_dev.sock

The socket option does not have a default value, and this option will not be used if empty. Both the bind_address and socket options can be defined at the same time. (Bug #21874593)