This PR builds on top of https://github.com/coder/coder/pull/25070 to
add a way of running the larger "fake agent" manager via the existing
CLI, pulling in the URL/credentials already set.
With this, we can run a pod per scaletest region to act as all the
workspaces in that region.
This is in a new subcommand `scaletest agentfake` currently.
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Signed-off-by: Callum Styan <callumstyan@gmail.com>