feat: return hostname suffix on AgentConnectionInfo (#17334)

Adds the Hostname Suffix to `AgentConnectionInfo` --- the VPN provider will use it to control the suffix for DNS hostnames.

part of: #16828

codersdk/workspacesdk/workspacesdk.go

@@ -143,6 +143,7 @@ type AgentConnectionInfo struct {
 	DERPMap                  *tailcfg.DERPMap `json:"derp_map"`
 	DERPForceWebSockets      bool             `json:"derp_force_websockets"`
 	DisableDirectConnections bool             `json:"disable_direct_connections"`
+	HostnameSuffix           string           `json:"hostname_suffix"`
 }
 
 func (c *Client) AgentConnectionInfoGeneric(ctx context.Context) (AgentConnectionInfo, error) {