mon/OSDMonitor: allow PG splitting for crimson pools by default

mon/OSDMonitor: allow PG splitting for crimson pools by default

Previously, crimson pools were created with FLAG_NOPGCHANGE hardcoded,
preventing any adjustment to pg_num since splitting and merging were
not yet supported.

This patch:
- Removes the hardcoded FLAG_NOPGCHANGE during crimson pool creation,
  and instead honors the global 'osd_pool_default_flag_nopgchange'.
- Introduces 'crimson_allow_pg_split' (default: true) to gate
  PG splitting.
- Explicitly blocks PG merging (shrinking) for crimson pools,
  as shrinking remains unsupported.

Signed-off-by: Aishwarya Mathuria <amathuri@redhat.com>