rpc: add SetHeader method to Client (#21392)

Resolves #20163

Co-authored-by: Felix Lange <fjl@twurst.com>

rpc/client.go

@@ -85,7 +85,7 @@ type Client struct {
 
 	// writeConn is used for writing to the connection on the caller's goroutine. It should
 	// only be accessed outside of dispatch, with the write lock held. The write lock is
-	// taken by sending on requestOp and released by sending on sendDone.
+	// taken by sending on reqInit and released by sending on reqSent.
 	writeConn jsonWriter
 
 	// for dispatch
@@ -260,6 +260,19 @@ func (c *Client) Close() {
 	}
 }
 
+// SetHeader adds a custom HTTP header to the client's requests.
+// This method only works for clients using HTTP, it doesn't have
+// any effect for clients using another transport.
+func (c *Client) SetHeader(key, value string) {
+	if !c.isHTTP {
+		return
+	}
+	conn := c.writeConn.(*httpConn)
+	conn.mu.Lock()
+	conn.headers.Set(key, value)
+	conn.mu.Unlock()
+}
+
 // Call performs a JSON-RPC call with the given arguments and unmarshals into
 // result if no error occurred.
 //