feat(dotnet): add C# 10 SDK, OpenFeature provider and NuGet releases - #35

Adds a C# 10 .NET SDK for applications that evaluate Flagship flags over HTTP. The new FlagshipServerProvider integrates with the official OpenFeature SDK to resolve boolean, string, integer, double, and object flags, preserving evaluation reasons, variants, and caller defaults.

• Two packages: Cloudflare.Flagship, with no external dependencies, and Cloudflare.Flagship.OpenFeature, using OpenFeature 2.14.1. Both target .NET 8 and .NET 10 with LangVersion=10.0.
• Async HTTP client with configurable endpoints, bearer authentication, static and dynamic headers, per-attempt timeouts, bounded retries, cancellation, and an injectable HttpClient that remains caller-owned.
• Culture-independent scalar context serialization, response validation, typed errors, and handling for disabled flags and type mismatches.
• README, compiled example, Changeset, xUnit v3 tests, and a CI job covering formatting, tests, and NuGet packaging. Both package versions are read from the package.json synchronized by the existing Changesets workflow.
• Automated nuget.org publication integrated with the existing Changesets release flow. Maintainers only need to configure the optional NUGET_API_KEY Actions secret with permission to publish both package IDs. Without it, the credential check succeeds and the publishing job is skipped.
• A sdks/dotnet/v<version> tag is created only after both package pushes succeed. Unpublished changes remain eligible across releases; --skip-duplicate supports retries after partial publication. Setup, credential scope, rotation, and recovery are documented in sdks/dotnet/PUBLISHING.md.

Local validation completed:

• pnpm run check, pnpm run changeset:validate, and pnpm run test passed, including 308 TypeScript tests and 23 release/publication tests, including missing credentials, partial upload failures, and publication baseline tracking.
• actionlint passed for the release and reusable NuGet workflows; the exact library packing commands used by the publishing workflow also passed locally.
• dotnet format Flagship.sln --verify-no-changes --no-restore passed.
• dotnet test Flagship.sln -c Release: 32 tests passed for each target (.NET 8 and .NET 10), covering integration through the official OpenFeature client, HTTP errors, retries, timeouts, cancellation, context serialization, and resource ownership.
• dotnet pack Flagship.sln -c Release --no-build -o artifacts produced both packages. Installation and flag evaluation were also verified in a separate application consuming the generated NuGet packages on both runtimes.

This contribution provides HTTP evaluation without response caching or native Workers bindings. Tests use a simulated HTTP transport and do not contact a live Flagship account. Publication scripts are tested with simulated commands; no packages have been uploaded to nuget.org during development. Live publication starts after maintainers configure the secret and run an eligible release.