Add support for running Relay as a process supervisor

[ezekg]

E.g. something like this in cases where only a single process/container is needed, or when another supervisor is involved:

relay serve --exec 'myapp -xy --z'

This would be an alternative to running Relay as a sidecar.

[ezekg]

I kind of like this style better, and it seems more conventional:

relay serve -p 4444 -- myapp --foo

I guess it all depends on how well Cobra supports --.