Add a WithMonologChannel attribute (#1847)

This attribute is meant to be used by frameworks / integrations to
choose which logger instance to inject in a class when they manage
several channels, if they decide to use it.
This attribute will have no effect in Monolog itself as the wiring of
the logger in other classes is not managed by Monolog.

tests/Monolog/Attribute/WithMonologChannelTest.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace Monolog\Attribute;
+
+use PHPUnit\Framework\TestCase;
+
+class WithMonologChannelTest extends TestCase
+{
+    public function test(): void
+    {
+        $attribute = new WithMonologChannel('fixture');
+        $this->assertSame('fixture', $attribute->channel);
+    }
+
+}