Read out heap space used and unallocated

[kristoftjonck]

Is your feature request related to a problem? Please describe.
I need to know how much heap size is allocated, how much is used and how much is left. Like the "xPortGetFreeHeapSize()" in freeRTOS.

Describe the solution you'd like
Function to read out the total heap size, used allocated heap space / unallocated heap space left.

Not sure if it is implemented, but could not find it in the documentation.

[aescolar]

CC @andyross @andrewboie

[carlescufi]

@andyross could you consider adding this to your new heap allocator?

[andyross]

Duplicate of #23076