Support new aggregation operators in builder (doctrine#2514)

* Add interfaces for aggregation pipeline operators

* Implement new aggregation pipeline operators in stages

* Update static analysis baselines

* Use consistent formatting for operators with optional arguments

* Use consistent capitalisation for "date object"

* Wrap docblock comments

* Rename ensureArray to something more descriptive

* Remove duplicated docblocks

* Use func_get_args when proxying expression methods

* Fix wrong return types for internal expr methods

* Use func_get_args for operators where possible

* Disambiguate closure param in tests

* Fix docblock errors

Note: ProvidesGroupAccumulatorOperators errors were ignored as the trait is designed to be used in conjunction with the corresponding interface.

* Make initArgs parameter for $accumulator optional

* Use static return types across aggregation builder

* Fix handling of optional parameters for $range and $slice

lib/Doctrine/ODM/MongoDB/Aggregation/Builder.php

@@ -361,7 +361,7 @@ public function group(): Stage\Group
     /**
      * Set which class to use when hydrating results as document class instances.
      */
-    public function hydrate(?string $className): self
+    public function hydrate(?string $className): static
     {
         $this->hydrationClass = $className;
 
@@ -524,7 +524,7 @@ public function replaceWith($expression = null): Stage\ReplaceWith
     /**
      * Controls if resulting iterator should be wrapped with CachingIterator.
      */
-    public function rewindable(bool $rewindable = true): self
+    public function rewindable(bool $rewindable = true): static
     {
         $this->rewindable = $rewindable;