Adds `--aigateway-proxy-target` option to
`deploymentGroupAIGatewayProxy` that defines URL to which intercepted
requests should be forwarded to.
Forward URL used to be hardcoded to `coderAPI.AccessURL` pointing to
embedded Gateway. With addition of standalone AI Gateway this needs to
be configurable.
Renamed `aibridgeproxyd.Server.coderAccessURL` and `coderAccessPort` ->
`gatewayURL` and `gatewayPort` + option to better reflect reality.