Include additional HTTP headers with the API request

[gcatanese]

This PR improves the README adding a section about passing HTTP request headers.
It also adds a test.

[gemini-code-assist]

Summary of Changes

Hello @gcatanese, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the client library by providing clear documentation and a corresponding test case for including custom HTTP headers and idempotency keys in API requests. This change improves the library's usability for developers who need to pass specific request options, ensuring that such functionality is both documented and validated.

Highlights

• Documentation Update: A new section has been added to the README.md file, detailing how to include additional HTTP headers and an idempotency key in API requests using the RequestOptions object.
• New Test Case: A new test, testRequestWithHttpHeaders, has been introduced in ClientTest.java to verify the correct application of custom headers and idempotency keys when making API requests.

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[gemini-code-assist]

Code Review

This pull request adds documentation and a test for including additional HTTP headers in API requests. The new section in the README provides a helpful example, but it contains a compilation error that needs to be fixed. I've also pointed out a minor formatting inconsistency in the README. The added test case is well-written and correctly verifies the new functionality.