Add CI workflow, update documentation, and convert examples to Go

[francescobianco]

This pull request introduces the initial setup and documentation for the Openapi Go SDK, establishing the project's structure, licensing, contribution guidelines, and CI/CD workflow. It also adds example code and unit tests for core functionality. The most important changes are summarized below.

Project setup and documentation:

• Added a comprehensive README.md outlining project overview, features, usage examples, installation instructions, testing, contribution guidelines, authors, partners, and licensing. [1] [2]
• Added LICENSE file with the MIT license, clarifying usage and distribution rights.
• Added docs/contributing.md and docs/code-of-conduct.md to guide contributors and maintain a welcoming environment. [1] [2]

Build and release tooling:

• Added Makefile with commands for testing, vetting, development pushes, and tagged releases, including GitHub release automation.
• Added .github/workflows/go.yml for GitHub Actions CI, running build and test jobs on pushes and pull requests.

Examples and testing:

• Added example code for token generation and API calls in examples/token_generation/token_generation.go and examples/api_calls/api_calls.go. [1] [2]
• Added unit tests for the client in pkg/client/client_test.go, covering initialization and request handling with and without query parameters.

Repository configuration:

• Updated go.mod to set the module path to github.com/openapi/openapi-go-sdk.
• Added .convcommit file to standardize commit messages using the convcommit CLI tool.