Added an `IsBuilt` property and `EnsureNotBuilt()` helper to `Factory` class
to restrict modifications to the service collection once the service provider
has been built. This ensures immutability and prevents runtime inconsistencies
after initialization.
- Introduced Factory class in DigitalData.Core.Abstractions namespace
- Implements both IServiceProvider and IServiceCollection interfaces
- Provides singleton instance via static readonly Instance property
- Supports lazy initialization of IServiceProvider using Lazy<T>
- Includes full collection management (Add, Remove, Clear, etc.)
- Ensures compatibility with both .NET and .NET Framework via conditional directives
