feat(core): Export a reusable function to add tracing headers (#20076)

This PR is an extraction of #19991 

It basically exports `getTracingHeadersForFetchRequest`, which was
previously only exported for testing, but offers a great functionality
if you want to add tracing headers to a request. I renamed it as
`addTracingHeadersToFetchRequest` sounded a little misleading, as it
didn't really add headers to the request, as it returned the extracted
headers from the request (or init, if there are any).

### Open question

I added `@hidden` and `@internal` to it, not sure if this is an approach
we follow. I'm ok to remove it from the jsdoc