#### Problem

We have too many pool allocators.

Previous PRs (project-chip#11428, project-chip#11487) transitionally use `BitMapObjectPool`
where previously `System::ObjectPool` had been used, but this lost
the ability to configure heap allocation.

#### Change overview

- Add a heap allocator (from project-chip#9590)
- Add allocation selection (from project-chip#11371)
- Use this for `System::Timer` (complementing project-chip#11487)

Co-authored-by: Zang MingJie <zealot0630@gmail.com>
Co-authored-by: C Freeman <cecille@google.com>

A future PR will use this for Inet pools (complementing project-chip#11428);
that is not done here because it would conflict with other Inet
changes under way.

#### Testing

Added heap versions of unit tests in TestPool. (A future PR will add
`System::Object`-style statistics and re-unify most of these tests.)

CI should show `.bss` decreases corresponding to increases in project-chip#11487.

- Factor out common code.
- Use a heap-allocated list to track heap-allocated objects.
- More unit tests.