Comprehensive In-Memory Caching (fix #404)

[mokarchi]

How It Works

When you call new ArchLoader().LoadAssemblies(...).Build(), ArchUnitNET automatically:

1. Creates a cache key based on the modules being loaded and any namespace filters applied
2. Checks the cache for a previously loaded Architecture instance matching this key
3. Returns the cached instance if found, or loads and caches a new instance if not

This means that loading the same assemblies multiple times (even with different ArchLoader instances) will reuse the cached Architecture object, providing dramatic performance improvements.

Usage Examples

Basic usage (automatic caching with file-based invalidation):

var arch = new ArchLoader()
    .LoadAssemblies(assembly)
    .Build(); // Automatically cached with invalidation

Custom cache key for test isolation:

var arch = new ArchLoader()
    .WithUserCacheKey("test-run-1")
    .LoadAssemblies(assembly)
    .Build();

Disable caching for specific scenarios:

var arch = new ArchLoader()
    .WithoutCaching()
    .LoadAssemblies(assembly)
    .Build();

Advanced configuration (config is cloned internally):

var config = new ArchLoaderCacheConfig
{
    CachingEnabled = true,
    UseFileBasedInvalidation = true, // Check file changes
    UserCacheKey = "integration-tests",
    IncludeVersionInCacheKey = true // Invalidate on version change
};

var arch = new ArchLoader()
    .WithCacheConfig(config)
    .LoadAssemblies(assembly)
    .Build();

// Safe: modifying config after passing it won't affect the ArchLoader
config.UserCacheKey = "different-key";

[mokarchi]

@alexanderlinne Could you review this, please?