remote-build: add support for `--build-for`

[sed-i]

What needs to get done

Like snapcraft, charmcraft should support the --build-for arg for remote build.

> charmcraft remote-build --build-for=s390x --launchpad-accept-public-upload
Usage: charmcraft [options] command [args]...
Try 'charmcraft remote-build -h' for help.

Error: unrecognized arguments: --build-for=s390x

Why it needs to get done

For testing a subset of charms, it would be handy if we didn't need to wait for all of them to be built remotely.
The amd64 and arm64 remotes are often busy, and github has amd64 and arm64 runners.
Building remotely can take an hour, vs. locally ~5 min.