Add gc and shutdown callbacks to ZendMM custom handlers

[realFlowControl]

Hello everyone 👋

This PR aims to add a gc() and shutdown() callback to the ZendMM custom heap (currently we only have alloc(), free() and realloc()). These two new functions would allow to do garbage collection and cleanup in a custom memory manager and/or allow to handle some side effects of using the zend_mm_set_custom_handlers() API when "just" observing the ZendMM (see #11758, "observing" as in passing calls back to ZendMM).

The initial idea was to add a zend_mm_set_custom_handlers_ex() function to only set the gc() and shutdown() hooks, but this has a major drawback: It would look like you could set gc and shutdown handlers without calling zend_mm_set_custom_handlers() and therefore we'd need to enforce calling the one before the other (which I'd like to refrain from doing) or it would raise complexity in handling cases where one set of handlers exists, while another does not. We could also add a use_custom_heap_ex flag, but I'd also like to refrain from doing this, as It would add additional checks in the hot path (during (de-)allocations).

I think it makes sense to have the gc and shutdown bundled with the other hooks. In the current version they are nullable, but I start thinking that they should not.

CC: @arnaud-lb

[dstogov]

I reviewed only zend_alloc part of the PR, and it looks almost fine.
See the comments about zend_mm_heap_create/zend_mm_heap_free calls.

[realFlowControl]

I did an implementation in https://github.com/realFlowControl/zendmmobserver_custom_handler that shows how to use this to build an observer without relying on tricky things like tinkering with the _zend_mm_heap->use_custom_heap flag which is at risk (see #14570)

Additionally this seems to allow to register in MINIT/MSHUTDOWN and not RINIT/RSHUTDOWN. For correct ZTS support GINIT and GSHUTDOWN should be the way to go though, I will have to play with this a bit.

[dstogov]

zend_alloc.* changes look good to me.

[realFlowControl]

@arnaud-lb in observe zendmm I played with it and with this PR, we can (un-)register ZendMM custom handlers in GINIT/GSHUTDOWN already without having any side effects from observing (forwarding back to the original heap). What is also pretty neat is that in case you want to stop observing, you can just call zend_mm_set_heap() with the original heap and if you want to start observing again, just call zend_mm_set_heap() with the observed heap.
I also ran some tests with ext-parallel and FrankenPHP with enabled observe zendmm and everything was stable 🎉

[arnaud-lb]

Thank you!