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:

[matifali]

Workspace Proxy is only used for Web apps. Each workspace proxy also hosts a DERP server, which Coder automatically uses if it can't establish a direct connection.

I don't think there's an easy way to allow users to choose their preferred DERP per connection.

@deansheather could you please help clarify my understanding and the issue request better? Thanks.