Add GetAllHeaders method to golang HTTP filter

[willemveerman]

Commit Message: Add GetAllHeaders method to golang HTTP filter
Additional Description: Adds a new method which returns a copy of the underlying map[string][]string object which contains all the request headers.
Risk Level: Low
Testing: Unit test
Docs Changes: none required - doesn't change the configuration params of the plugin
Release Notes:
Platform Specific Features:
[Optional Runtime guard:]
[Optional Fixes #Issue]
[Optional Fixes commit #PR or SHA]
[Optional Deprecated:]
[Optional API Considerations:]

[repokitteh-read-only]

Hi @willemveerman, welcome and thank you for your contribution.

We will try to review your Pull Request as quickly as possible.

In the meantime, please take a look at the contribution guidelines if you have not done so already.

🐱

Caused by: #33821 was opened by willemveerman.

see: more, trace.

[repokitteh-read-only]

As a reminder, PRs marked as draft will not be automatically assigned reviewers,
or be handled by maintainer-oncall triage.

Please mark your PR as ready when you want it to be reviewed!

🐱

Caused by: #33821 was opened by willemveerman.

see: more, trace.

[doujiang24]

Thanks, we need some tests though the implementation is simple.

[phlax]

@doujiang24 i think this may be waiting for further review

[willemveerman]

@doujiang24 i think this may be waiting for further review

I still need to add the tests

[willemveerman]

@doujiang24 I've added what I think is a test, but I'm not sure how to run the tests - can you shed some light?

[doujiang24]

@willemveerman Here is a doc section: Testing Envoy with Bazel

For the current ci failure:

compilepkg: missing strict dependencies:
	/b/f/w/contrib/golang/filters/http/test/test_data/basic/filter.go: import of "golang.org/x/exp/slices"

https://dev.azure.com/cncf/envoy/_build/results?buildId=170352&view=logs&j=8c169225-0ae8-53bd-947f-07cb81846cb5&t=d1a98671-b7ba-5fbf-f06c-ff337c010df4&l=159

As the error message shows it missing dependency.
it would be a easier way to remove such dependency, it might be a bit complicated to introduce a golang dependency in bazel.

[willemveerman]

@doujiang24 OK, it's ready for review now

Actually, I'm just going to improve the tests a little

[willemveerman]

OK I've added one further test to cover the case of the headerMap being changed, in order to verify that the slices are being copied. Sorry for delay, I had a hard time building the tests on my m2 arm, but found a workaround. @doujiang24

[doujiang24]

just a small style issue, otherwise, lgtm, thanks~

[doujiang24]

lgtm, thanks