fix(pdk): get the first http header value after bumping to OpenResty 1.25.3.1

[chronolaw]

Summary

It is a fixing PR for #12327.
KAG-3570

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

Fix #[issue number]

[bungle]

@chronolaw we need to somehow also handle the header modification case too?

This is how our current code works I think:

kong.service.request.set_header("a", "b")
local a = kong.request.get_header("a")
kong.service.request.set_header("a", "c")
local a2 = kong.request.get_header("a")
assert(a ~= a2) 

It would though be possible with this new ctx thing to have client request headers separated from upstream request headers, but that is not what we currently do, but something to think about perhaps, we already have:

kong.service.response.get_header()
kong.response.get_header()

That do separate service response headers from kong response headers (kong response headers sure contain the upstream response headers too, but they may contain modifications/additions too).

Perhaps the cache thing needs to be in those global patches.

[chronolaw]

If we have an assumption that multiple header value is not a frequent case when accessing by ngx.var,
could we not cache the result of get_headers() in ctx table?

changed accordingly.

[team-gateway-bot]

Cherry-pick failed for master, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally.

git remote add upstream https://github.com/kong/kong-ee
git fetch upstream master
git worktree add -d .worktree/cherry-pick-12370-to-master-to-upstream upstream/master
cd .worktree/cherry-pick-12370-to-master-to-upstream
git checkout -b cherry-pick-12370-to-master-to-upstream
ancref=$(git merge-base 58fe2dd31c06a1f2910c2b69deb678f39c573177 1198c8132c4009849eeaec9bdbf134f13eab767b)
git cherry-pick -x $ancref..1198c8132c4009849eeaec9bdbf134f13eab767b

[chronolaw]

Only when the cherry-pick of #12327 is merged we will cherry pick this one.

[team-gateway-bot]

Cherry-pick failed for master, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally.

git remote add upstream https://github.com/kong/kong-ee
git fetch upstream master
git worktree add -d .worktree/cherry-pick-12370-to-master-to-upstream upstream/master
cd .worktree/cherry-pick-12370-to-master-to-upstream
git checkout -b cherry-pick-12370-to-master-to-upstream
ancref=$(git merge-base 58fe2dd31c06a1f2910c2b69deb678f39c573177 1198c8132c4009849eeaec9bdbf134f13eab767b)
git cherry-pick -x $ancref..1198c8132c4009849eeaec9bdbf134f13eab767b