Fix return type in abstract methods from self to static (#53)

* Change return type of abstract request methods

* Change return type of `fromParams` method to `static` in request classes

(cherry picked from commit 94799ab66ff60d4b2305aadb6b73aaa4c805221e)

* Make request classes final and update `getParams` return types for improved type safety

(cherry picked from commit d6be496f89cc0e6d30f519792d0dc902685c4178)

tests/Schema/JsonRpc/RequestTest.php

@@ -24,7 +24,7 @@ final class RequestTest extends TestCase
                 return 'foo/bar';
             }
 
-            public static function fromParams(?array $params): self
+            public static function fromParams(?array $params): static
             {
                 return new self();
             }