Support plugin path(s) variable in config file

What

Support a plugin_paths variable in the OpenAssetIO config TOML, which (effectively) appends to OPENASSETIO_PLUGIN_PATH.

Why

It would reduce the setup steps for enabling a host to work with a particular manager if they only had to set one environment variable (OPENASSETIO_DEFAULT_CONFIG) instead of two.

In combination with the built-in ${config_dir} expansion, this would mean the config and plugin(s) could live together in a relocatable way.

Acceptance Criteria

The environment variable would take precedence over the config file.
platform_paths should be a TOML list
We should tolerate non existent paths, and just print a warning (which is probably already the case). This allows config files to be shared between hosts/platforms.
Needs to support C++ and Python