Add PHPDoc return types

Symfony PHPUnit Bridge has a deprecation helper that warns when things might break in the future.

It currently reports the following errors when using Stripe PHP SDK:

```
Method "ArrayAccess::offsetExists()" might add "bool" as a native return type declaration in the future. Do the same in implementation "Stripe\StripeObject" now to avoid errors or add an explicit @return annotation to suppress this message.
Method "ArrayAccess::offsetGet()" might add "mixed" as a native return type declaration in the future. Do the same in implementation "Stripe\StripeObject" now to avoid errors or add an explicit @return annotation to suppress this message.
Method "ArrayAccess::offsetSet()" might add "void" as a native return type declaration in the future. Do the same in implementation "Stripe\StripeObject" now to avoid errors or add an explicit @return annotation to suppress this message.
Method "ArrayAccess::offsetUnset()" might add "void" as a native return type declaration in the future. Do the same in implementation "Stripe\StripeObject" now to avoid errors or add an explicit @return annotation to suppress this message.
Method "Countable::count()" might add "int" as a native return type declaration in the future. Do the same in implementation "Stripe\StripeObject" now to avoid errors or add an explicit @return annotation to suppress this message.
Method "JsonSerializable::jsonSerialize()" might add "mixed" as a native return type declaration in the future. Do the same in implementation "Stripe\StripeObject" now to avoid errors or add an explicit @return annotation to suppress this message.
```

By adding these PHPDoc types, the problem disappears. It's also more clear for users of the package.

Fixes #1210

lib/StripeObject.php

@@ -218,7 +218,9 @@ public function offsetGet($k)
         return \array_key_exists($k, $this->_values) ? $this->_values[$k] : null;
     }
 
-    // Countable method
+    /**
+     * @return int
+     */
     #[\ReturnTypeWillChange]
     public function count()
     {
@@ -424,6 +426,9 @@ public function serializeParamsValue($value, $original, $unsaved, $force, $key =
         }
     }
 
+    /**
+     * @return mixed
+     */
     #[\ReturnTypeWillChange]
     public function jsonSerialize()
     {

lib/Util/CaseInsensitiveArray.php

@@ -33,6 +33,9 @@ public function getIterator()
         return new \ArrayIterator($this->container);
     }
 
+    /**
+     * @return void
+     */
     #[\ReturnTypeWillChange]
     public function offsetSet($offset, $value)
     {
@@ -44,6 +47,9 @@ public function offsetSet($offset, $value)
         }
     }
 
+    /**
+     * @return
+     */
     #[\ReturnTypeWillChange]
     public function offsetExists($offset)
     {
@@ -52,13 +58,19 @@ public function offsetExists($offset)
         return isset($this->container[$offset]);
     }
 
+    /**
+     * @return void
+     */
     #[\ReturnTypeWillChange]
     public function offsetUnset($offset)
     {
         $offset = static::maybeLowercase($offset);
         unset($this->container[$offset]);
     }
 
+    /**
+     * @return mixed
+     */
     #[\ReturnTypeWillChange]
     public function offsetGet($offset)
     {