chore: add devcontainer support

[askpt]

This PR

This pull request introduces support for Development Containers (devcontainers) to the repository, making it easier for developers to set up a consistent, containerized development environment. The main changes include the addition of a devcontainer configuration file and documentation updates explaining how to use devcontainers with Visual Studio Code and GitHub Codespaces.

Devcontainer support:

• Added .devcontainer/devcontainer.json with configuration for the development environment, including the .NET SDK, useful VS Code extensions, and required features.

Documentation improvements:

• Updated CONTRIBUTING.md to include instructions for using devcontainers, detailing prerequisites and steps for setup with VS Code and GitHub Codespaces.

Related Issues

Fixes #574

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.04%. Comparing base (fdf2297) to head (b7bbbb8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #584   +/-   ##
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  Coverage   90.04%   90.04%           
=======================================
  Files          77       77           
  Lines        2873     2873           
  Branches      323      323           
=======================================
  Hits         2587     2587           
  Misses        226      226           
  Partials       60       60           

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[Copilot]

Pull Request Overview

This PR adds Development Container (devcontainer) support to enable a consistent, containerized development environment for the OpenFeature .NET SDK project.

• Added devcontainer configuration with .NET SDK 9.0/8.0 and useful VS Code extensions
• Updated contributing documentation with setup instructions for VS Code and GitHub Codespaces

Reviewed Changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 2 comments.

File	Description
.devcontainer/devcontainer.json	New devcontainer configuration with .NET runtime, development tools, and VS Code extensions
CONTRIBUTING.md	Added devcontainer setup instructions and prerequisites

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