Make dns available through sansshell proxy.

[sfc-gh-mwalas]

At least during local development followinghttps://github.com/Snowflake-Labs/sansshell?tab=readme-ov-file#build-and-run the DNS resolution fails when calling out through proxy

$ go run ./cmd/sanssh --proxy=localhost:50043 --targets=localhost:50042 dns lookup www.example.com
Dns lookup failure for target localhost:50042 (0) - error - rpc error: code = Internal desc = got reply error from stream. Code: InvalidArgument Message: unknown method /Dns.Lookup/Lookup

while it succeeds when contacting server directly

go run ./cmd/sanssh --targets=localhost dns lookup www.example.com
93.184.215.14

I suspect this is due to lack of linking of dns in

sansshell/cmd/proxy-server/main.go

Line 47 in c38af02

// Import services here to make them proxy-able