Add keepalive support in the client

[stgraber]

This adds a new incus remote proxy sub-command which operates a simple reverse proxy server over a unix socket. This then gets used when a keepalive property is set on a remote.

That property is a number of seconds during which to keep the proxy running.

Using the keepalive proxy has a few advantages:

• Allows for more efficient backend connections (keep-alive)
• Keeps OIDC initialized over multiple calls
• Caches the calls to /1.0, effectively reducing API calls in half

In my tests, this leads to around 30% performance improvement on workloads that run a lot of individual CLI calls like Ansible.