New X-ElasticPress-Request-ID header

[felipeelia]

Description of the Change

Closes #3305

How to test the Change

Changelog Entry

Added - Requests now have a new header called X-ElasticPress-Request-ID to help with debugging.

Credits

Props @felipeelia, @dustinrue

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[felipeelia]

I am merging this as is, after a conversation with Jacob via slack. The main reason why the generateRequestId isn't in the API is that it is also used by autosuggest, which does not use an API-driven approach yet. When it comes, we can bring that function in, and fix the API encapsulation again.