Devcontainers: Download sub agent binaries and support different architectures than host workspace

The ticket #621 was addressed by coder/coder#18245, but one task was broken out into this new as lower priority:

- Download and cache agent binaries based on platform requirements of dev container (if different than host)

Additionally worth considering:

- If the coderd agent binaries are newer than the host workspace agent (e.g. after coderd upgrade), do we download an updated binary even when host and container architecture matches?