Add caching layer for TenantFinder

[moisish]

Summary

Tenant resolution via TenantFinder hits the database on every request. This PR adds an opt-in caching decorator that wraps any TenantFinder implementation and caches the resolved tenant.

What's included

• CachingTenantFinder — a decorator that wraps the configured TenantFinder, caches the result, and delegates to the inner finder on cache miss. Supports configurable TTL, cache store, and forever-cache (ttl: 0). Null results are intentionally not cached.
• CachesTenantFinderResults — interface exposing forget(Request) and forgetByKey(string) for cache invalidation, allowing other packages to type-hint without depending
  on the concrete class.
• Config-driven — disabled by default. Users enable it and optionally swap the class for custom cache key strategies (e.g. subdomain-based resolution):

'tenant_finder_cache' => [
    'enabled' => false,
    'class' => CachingTenantFinder::class,
    'store' => null,
    'ttl' => 300, // seconds, 0 = forever
],

• Extensible — override resolveCacheKey() or cachePrefix() for custom cache key derivation:

 class SubdomainCachingTenantFinder extends CachingTenantFinder
 {
     protected function resolveCacheKey(Request $request): string
     {
         return $this->cachePrefix() . explode('.', $request->getHost())[0];
     }
 }

[masterix21]

Hey @moisish, thanks for the PR!

Since TenantFinder is fully swappable via config, you can add a caching layer in your own implementation without needing changes to the package itself — keeping things flexible per project. Also, the test suite is currently failing, so we can't merge this as-is.

Thanks again for the contribution, we appreciate it!