feat(devtools): add Swagger and gRPC developer tools plugin

[rian-be]

Summary

This PR adds the DevTools plugin for serving Swagger UI and providing web interface for browsing and invoking gRPC services dynamically from protobuf descriptors.

DevTools Plugin

• adds the DevTools plugin project and runtime registration
• exposes Swagger UI and the generated OpenAPI document
• adds a browser UI for inspecting gRPC services and methods
• supports dynamic unary gRPC invocation without generated client stubs

Runtime Service Catalog

• discovers gRPC services from registered server descriptors
• exposes service, method, request, and response metadata
• supports dynamic invocation of unary methods
• returns structured invocation results and errors

Configuration

• adds DevTools options and Swagger UI options
• supports configurable route prefix and document name
• integrates DevTools into the solution and Docker image
• publishes DevTools as a dynamically loaded plugin

Documentation

• adds ADR-020 for the DevTools plugin
• adds ADR-021 for Swagger serving through reflection

Validation

• editor diagnostics are clean for the affected Host and DevTools files
• git diff --check passes
• full restore and test execution require the .NET 10 SDK

Result

AuthKit can now expose developer tooling as a separate dynamically loaded plugin while keeping Swagger generation in the Host and gRPC inspection/invocation in DevTools.

Closes #33