Use generics to precise User class

[VincentLanglet]

Hi @bocharsky-bw

These generics are useful for static analysis like PHPStan or psalm, because it allows to precise the return type or the expected param of some methods (instead of a general object one).

The only failure is

1) src/Model/ResetPasswordRequestTrait.php
      ---------- begin diff ----------
--- /home/runner/work/reset-password-bundle/reset-password-bundle/src/Model/ResetPasswordRequestTrait.php
+++ /home/runner/work/reset-password-bundle/reset-password-bundle/src/Model/ResetPasswordRequestTrait.php
@@ -52,7 +52,7 @@
     protected $expiresAt;
 
     /** @return void */
-    protected function initialize(\DateTimeInterface $expiresAt, string $selector, string $hashedToken)
+    protected function initialize(\DateTimeInterface $expiresAt, string $selector, string $hashedToken): void
     {
         $this->requestedAt = Clock::get()->now();
         $this->expiresAt = $expiresAt;

which is unrelated and already exists.

[VincentLanglet]

Friendly ping @bocharsky-bw Should i fix the existing CS failure too ?