- make it a GObject so that it can emit its own signals
and so that it can be shared between multiple proxies
- share the WpProps instance between endpoints, endpoint-streams
and their underlying nodes
- introduce the concept of the caching mode that redirects _set
to _set_param of the proxy that actually has the props; this allows
shared WpProps to actually set changes on the correct proxy
in a transparent way
- change methods to consume the ref of the pod and reflect that
also on wp_proxy_set_prop()
- refactor the export process on endpoints & endpoint-streams
so that they always get all the required features (info, props, bound)
and make it async so that we can take time to prepare the underlying
node to have FEATURE_PROPS
- update the props & endpoint unit tests, bringing back all the
checks that the endpoint unit test used to have

+ better logs by default
+ allow loading modules in tests

including wp/spa-props.c currently re-defines G_LOG_DOMAIN
in the spa-props.c unit test

+ enable the new log writer on the executables
+ enable structured logging in the tests

This is an asynchronous operation class, like GTask,
but it is made to execute several operations underneath,
using a state machine, instead of just a single operation.

heavily based on the WpSession implementation

This also adds a "test server" helper, which is used to
start a local pipewire server in the same process, for testing
functionality that requires a server.