Check for write access to remote artifact cache early on in the pipeline

Previously, the first time you configured an artifact cache, you would get to the end of your first build and then BuildStream would exit because of some stupid mistake like you got the address slightly wrong or you forgot to add the host keys of the remote artifact cache to ~/.ssh/known_hosts.

To avoid surprises, if there's an artifacts push-url configured we now try to connect to it as a preflight check so that issues are raised early.

As a bonus, for some reason this check causes SSH to ask about unknown host keys rather than just failing, so you may now see messages like this if the host keys are unknown rather than an error:

The authenticity of host '[ostree.baserock.org]:22200 ([185.43.218.170]:22200)' can't be established.
ECDSA key fingerprint is SHA256:mB+MNfYREOdRfp2FG6dceOlguE/Skd4QwnS0tvCPcnI.
ECDSA key fingerprint is MD5:8f:fa:ab:90:19:31:f9:f7:f1:d4:e5:f0:a2:be:56:71.
Are you sure you want to continue connecting (yes/no)?