feat: add --aigateway-proxy-target flag (#27122)

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.

cli/testdata/coder_server_--help.golden

@@ -258,6 +258,11 @@ AI GATEWAY PROXY OPTIONS:
           Path to the TLS private key file for the AI Gateway Proxy listener.
           Must be set together with AI Gateway Proxy TLS Certificate File.
 
+      --ai-gateway-proxy-target string, $CODER_AI_GATEWAY_PROXY_TARGET
+          Base URL of the AI Gateway to forward intercepted requests to.
+          Defaults to the embedded AI Gateway address at the Coder access URL
+          plus /api/v2/ai-gateway.
+
       --ai-gateway-proxy-upstream string, $CODER_AI_GATEWAY_PROXY_UPSTREAM
           URL of an upstream HTTP proxy to chain tunneled (non-allowlisted)
           requests through. Format: http://[user:pass@]host:port or