feat: Sanitise headers to OTel semconv and add as separate fields/attributes

[MikeGoldsmith]

Which problem is this PR solving?

The OTel semantic conventions for headers is to lowercase and replace - with _ and for each header to be a separate field/attribute. The logic to do the sanitisation was added in #290 for just the OTel handler.

This PR moves it to a utility func that both the OTel and libhoney handlers can use.

Short description of the changes

• Move sanitisation logic to shared utility func
• Use new func in both otel and libhoney handlers
• Update both handler tests
• Add -count=1 to test command to clear cache on test runs

How to verify that this has the expected result

Both libhoney and OTel handlers sanitise headers and add each header as a separate field / attribute.

[JamieDanielson]

I added -count=1 to test command to clear cache on test runs, and updated the PR description as it failed on my minor change and it shouldn't have. This is the idiomatic way to clear cache on test runs.

[JamieDanielson]

Looks great!
Here's an example with HTTP_HEADERS set to "User-Agent,Accept,Content-Type"