feat(spicedb): add testcontainer module for SpiceDB

[adborroto]

What does this PR do?

This PR adds a new SpiceDB module to the Testcontainers for .NET library, providing seamless integration for testing applications that use SpiceDB (Google Zanzibar-inspired permissions database).

Key components added:

• SpiceDBContainer: Container wrapper with SpiceDB-specific functionality including connection string generation
• SpiceDBBuilder: Fluent builder for configuring SpiceDB containers with sensible defaults
• SpiceDBConfiguration: Configuration class following the established Testcontainers pattern
• Comprehensive test suite: Full test coverage with container lifecycle, port mapping, and command execution tests
• Documentation: Complete module documentation following the established format used by other modules

Technical details:

• Uses authzed/spicedb:v1.45.1 Docker image
• Exposes gRPC API on port 50051
• Implements wait strategy using SpiceDB CLI ping command
• Follows the standard Testcontainers .NET module patterns and conventions

Why is it important?

SpiceDB is gaining significant adoption as a robust authorization service, especially for applications requiring fine-grained permissions at scale. This module enables developers to:

1. Test authorization logic reliably: Write integration tests against a real SpiceDB instance without complex setup
2. Improve development velocity: Spin up SpiceDB containers effortlessly in test environments
3. Ensure consistency: Test against the same SpiceDB version used in production
4. Simplify CI/CD: No need for external SpiceDB dependencies in testing pipelines

This addition expands Testcontainers' coverage into the authorization/permissions domain, complementing the existing database and messaging modules.

Related issues

• Relates to expanding Testcontainers module ecosystem
• Addresses community requests for SpiceDB testing support

How to test this PR

Prerequisites:

• Docker installed and running
• .NET 9.0 SDK

Testing steps:

1. Build the solution:

   dotnet build

2. Run SpiceDB tests specifically:

   dotnet test tests/Testcontainers.SpiceDB.Tests/

3. Run all tests to ensure no regressions:

   dotnet test

4. Verify documentation builds correctly:

   # Check that the documentation is properly formatted
   cat docs/modules/spicedb.md

Expected test results:

• ✅ Container starts successfully
• ✅ gRPC port (50051) is properly mapped
• ✅ SpiceDB commands execute successfully
• ✅ Container lifecycle management works correctly

Follow-ups

Potential enhancements for future PRs:

1. Enhanced connection string implementation: Currently returns null - should return proper gRPC connection string format
2. SpiceDB-specific helper methods: Add methods for schema management, relationship operations
3. Configuration presets: Add builder methods for common SpiceDB configurations (development, testing, etc.)
4. Client library integration examples: Demonstrate usage with popular SpiceDB client libraries
5. Performance testing utilities: Add helpers for load testing authorization scenarios

Documentation improvements:

• Add code examples showing integration with SpiceDB client libraries
• Include

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