feat: add enforce() and g() cache to pycasbin

[Doslam]

This PR adds CachedEnforcer, SyncedCachedEnforcer, AsyncCachedEnforcer and g() function cache for pycasbin. Three new enforcer classes are now available — CachedEnforcer (single-thread), SyncedCachedEnforcer (thread-safe) and AsyncCachedEnforcer (async-safe) — all providing decision caching on enforce() results. Existing Enforcer also gains enable_g_function_cache() to control role-lookup caching.

This aligns pycasbin with go-casbin's CachedEnforcer. Additionally, SyncedCachedEnforcer and AsyncCachedEnforcer are provided for concurrent workflows not covered by go-casbin.

Two independent caches:

• Decision cache — caches enforce() results. Repeated same sub/obj/act return instantly. Auto-invalidated on any policy mutation (_add_policy/_remove/_update/load/clear).
• g() function cache — caches has_link() on RoleManager via lru_cache. Avoids redundant role-graph traversal inside each enforce(). Opt-in via enable_g_function_cache(True). Cleared on add_link/delete_link/clear.

Cache invalidation

Operation	Decision cache	g() cache
add/remove/update p policy	Cleared	Unchanged
add/remove grouping policy	Cleared	Cleared
load_policy / clear_policy	Cleared	Cleared

Files

New:

• casbin/cache/init.py — Cache ABC
• casbin/cache/default_cache.py — DefaultCache (dict + TTL)
• casbin/cached_enforcer.py — CachedEnforcer
• casbin/synced_cached_enforcer.py — SyncedCachedEnforcer
• casbin/async_cached_enforcer.py — AsyncCachedEnforcer
• tests/test_cached_enforcer.py — 22 tests

Modified:

• casbin/core_enforcer.py — add enable_g_function_cache()
• casbin/rbac/default_role_manager/role_manager.py — lru_cache + clear_g_cache + DomainManager propagation
• casbin/init.py — exports

Compatibility

Zero breaking changes. g() cache disabled by default. All additive.