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: local manual testing
Docs Changes: none required (only changes the capabilities afforded to the plugin, does not change envoy itself)
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

[doujiang24]

this won't clone a slice too.

[willemveerman]

I don't fully understand, can you elaborate?

[doujiang24]

it is shallow copy now, here is an example:

	// slice
	s := []int{1, 2, 3}
	d := s
	s[0] = 4
	println(d[0]) // 4

I think we need deep clone for safety, thoughts?

[willemveerman]

I see what you mean.... This should do what we need

	for key, value := range h.headers {
		copied_headers[key] = append(value[:0:0], value...)
	}

Or we could do this

	for key, value := range h.headers {
		copied_headers[key] = make([]string, len(value))
		copy(copied_headers[key], value)
	}

[willemveerman]

The second option is more readable imo