Refactor trace header support

[costela]

Following up from the discussion in #57 (comment)

[riandyrn]

Hello, @Costella

Thanks for your new PR, and I'm sorry for the late review.

However, I noticed in this PR, you are trying to do several things:

1. Implement WithTraceResponseHeaders() option & deprecate WithTraceIDResponseHeader.
2. Refactor config by making all the properties private.
3. Refactor sdk_test.go by adding t.Helper() to several test cases.

While it is tempting to implement all of these in one go, let's only focus on implementing the first objective. This will make the review much easier and ensure we won't run into unintended consequences.

cc: @ilhamsyahids

[costela]

@riandyrn yes, that's reasonable! Just removed most the rest of the changes. Will split them into another PR for consideration.
Kept the two t.Helper instance that are actually called in the new test, since they are arguably part of the change. WDYT?

[costela]

Just a friendly ping, in case anyone happens to have some time for a review this week 😇

[costela]

the pipeline failure seems unrelated? 🤔

[ilhamsyahids]

LGTM, thanks a lot @costela! 🔥🚀

[ilhamsyahids]

Not sure about the pipeline, let me try on different branch