[FR] Ability to choose underlying containers for flat maps + extended allocators support

[psiha]

Similarily to the way Boost.Container does it:
https://www.boost.org/doc/libs/1_84_0/doc/html/container/extended_allocators.html
adopt_sequence

At the same time please try to accommodate "unusual backing storage": support using memory mapped, persistent, file backed storage - boostorg/container#260.