Layer2/Store: indexed store functors

Context

This MR introduces functors to build indexed stores (with files and indexes) for Layer 2 nodes.

Manually testing the MR

$ dune exec src/lib_layer2_store/test/test_indexed_store.exe -- -v
random seed: 460633412
generated error fail pass / total     time test name
[✓] 2000    0    0 2000 / 2000     7.0s singleton store (sequential)
[✓] 2000    0    0 2000 / 2000     3.6s indexable store (sequential)
[✓] 2000    0    0 2000 / 2000     3.6s indexable store (parallel)
[✓] 2000    0    0 2000 / 2000     2.8s indexable removable store (sequential)
[✓] 2000    0    0 2000 / 2000     2.7s indexable removable store (parallel)
[✓] 2000    0    0 2000 / 2000     3.7s indexed file store (sequential)
[✓] 2000    0    0 2000 / 2000     3.6s indexed file store (parallel)
================================================================================
success (ran 7 tests)