Support full header context with read-only on encode phase

[ardikabs]

Title: Support full header context with read-only on encode phase

Description:
This request pertains specifically to the Golang extension, which by now if by any chance the Decode phase is skipped (unreached), thus when it comes to the Encode phase, there will be no information related to the request flowing during the Encode phase, such as but not limited to when we need to know which host associated to the response.

Requesting to add support for read-only header map during phases that have not yet been reached.

[optional Relevant Links:]
Slack thread: https://envoyproxy.slack.com/archives/C04QNSXC7U0/p1714279861985839

[ardikabs]

/cc @doujiang24

[doujiang24]

yep, we could add an getRequestHeaders() method in the StreamInfo interface.
just like the C++ API: https://github.com/envoyproxy/envoy/blob/main/envoy/stream_info/stream_info.h#L858

I'll have a try after #33377

[github-actions]

This issue has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in the next 7 days unless it is tagged "help wanted" or "no stalebot" or other activity occurs. Thank you for your contributions.

[doujiang24]

@phlax please help to add an no stalebot label, thanks~

[ardikabs]

now that #34810 has been merged, is this request still relevant @doujiang24?

[doujiang24]

@ardikabs yes, let's keep it open.