Accept and validate a user-provided project name for remote build

[lengau]

What needs to get done

This requires https://github.com/canonical/craft-application/issues/274

Remote build should accept a user-provided project name. The project name will be selected as follows:

1. --launchpad-project=<name> in the command line
2. Fallback to CRAFT_REMOTE_BUILD_PROJECT environment variable
3. Get a project name from snap settings
4. Fall back to default

If any project name other than the default is provided through mechanisms 1-3, remote build should fail if the project does not exist or the user does not have correct permissions on it. Users wanting to use a non-default project must manage that project outside of snapcraft.

Why it needs to get done

This enables private remote build by use of private projects.