feat: allow DERP headers to be set (#6572)

* feat: allow DERP headers to be set

* chore: remove custom flag

* Clone DERP header on client create

* Adjust to use interface to cast headers

---------

Co-authored-by: Kyle Carberry <kyle@carberry.com>

codersdk/workspaceagents.go

@@ -143,9 +143,17 @@ func (c *Client) DialWorkspaceAgent(ctx context.Context, agentID uuid.UUID, opti
 	}
 
 	ip := tailnet.IP()
+	var header http.Header
+	headerTransport, ok := c.HTTPClient.Transport.(interface {
+		Header() http.Header
+	})
+	if ok {
+		header = headerTransport.Header()
+	}
 	conn, err := tailnet.NewConn(&tailnet.Options{
 		Addresses:      []netip.Prefix{netip.PrefixFrom(ip, 128)},
 		DERPMap:        connInfo.DERPMap,
+		DERPHeader:     &header,
 		Logger:         options.Logger,
 		BlockEndpoints: options.BlockEndpoints,
 	})