allow selection of workspace proxy

[dannykopping]

We deploy a set of workspace proxies around the world with a certain service provider.
That provider recently had an outage, and this appears to have lead to higher-than-normal latency on the proxy closest to me (Johannesburg). The proxy with the lowest latency doesn't seem to be chosen, nor can I configure it.

As a user I expected to be able to control this since I can select my preferred proxy in the Coder dashboard: