os/bluestore: support main/slow device's alloc unit for BlueFS.

os/bluestore: support main/slow device's alloc unit for BlueFS.

This effectively enables having 4K allocation units for BlueFS.
But it doesn't turn it on by default for the sake of performance.
Using main device which lacks enough free large continuous extents
might do the trick though.

Signed-off-by: Igor Fedotov <igor.fedotov@croit.io>
(cherry picked from commit 001b08d0b755a855f19f15b84104c7eb3d367c60)

 Conflicts:
	src/os/bluestore/BlueFS.cc
(trivial, no https://github.com/ceph/ceph/pull/39871/)
	src/os/bluestore/BlueStore.cc
(trivial, no commits for zoned support)
	src/test/objectstore/test_bluefs.cc
(trivial, no https://github.com/ceph/ceph/pull/45883)