Plugins: Improve hook performance by using `spl_object_id()` instead of `spl_object_hash()` to construct unique IDs.

* Also use `spl_object_id()` similarly when registering and unregistering classic widgets.
* Improve typing and phpdoc in `_wp_filter_build_unique_id()`. Return `null` for malformed callbacks.
* Add tests for `_wp_filter_build_unique_id()`.
* Improve type safety of `WP_Hook::add_filter()` in case an invalid callback is provided for parity with `::has_filter()` and `::remove_filter()`.

Developed in https://github.com/WordPress/wordpress-develop/pull/11865
Follow-up to r46220, r46801, r60179.

Props bor0, westonruter, SergeyBiryukov, schlessera, arshidkv12, knutsp, spacedmonkey, swissspidy.
See #64898.
Fixes #58291.


git-svn-id: https://develop.svn.wordpress.org/trunk@62408 602fd350-edb4-49c9-b593-d223f7449a82