ext_proc: template gRPC client and move it to common folder

[botengyao]

This PR will make the gRPC external processor client in l7_ext_proc share with the l4_ext_proc filter.

Commit Message:
Additional Description:
Risk Level: low
Testing:
Docs Changes:
Release Notes:

[repokitteh-read-only]

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

🐱

Caused by: #38898 was opened by botengyao.

see: more, trace.

[botengyao]

/retest

[repokitteh-read-only]

CC @envoyproxy/coverage-shephards: FYI only for changes made to (test/per_file_coverage.sh).
envoyproxy/coverage-shephards assignee is @alyssawilk

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Caused by: #38898 was synchronize by botengyao.

see: more, trace.

[RyanTheOptimist]

coverage LGTM

[yanjunxiang-google]

LGTM in general. Thanks for working on this refactoring!
Left a few nit comments. It's okay if you want to address it in the follow up PR.

[botengyao]

thanks for the review, done.

[tyxia]

Templates are great to provide flexible interfaces but it also will make interface/code more complicated. As we can see in this PR , template arguments are passed around/specified through the multiple layers in the stack.

Some high level comments:

1. Do you have the e2e working prototype of using this interface in L4 ext_proc?
2. Can (or will it be possible that) this gRPC client be generic enough to be shared with clients other than ext_proc?
3. Have you explored the techniques like CTAD; default template argument etc to reduce template specification?

It seems to be a major change, I don't have strong opinion if 1 above is working

[botengyao]

Templates are great to provide flexible interfaces but it also will make interface/code more complicated. As we can see in this PR , template arguments are passed around/specified through the multiple layers in the stack.

Some high level comments:

1. Do you have the e2e working prototype of using this interface in L4 ext_proc?

2. Can (or will it be possible that) this gRPC client be generic enough to be shared with clients other than ext_proc?

3. Have you explored the techniques like CTAD; default template argument etc to reduce template specification?

It seems to be a major change, I don't have strong opinion if 1 above is working

Thanks Tianyu for the review. yes it will work with the new l4 ext_proc, and I think this can be shared for other cases as well for different message types.

Good to know CTAD, I will explore it, can it be a follow up? Thanks!

[tyxia]

Follow-up actions are fine to me.
it will be great to reduce the code complexity (i.e., have easy to maintain code) while leveraging's template's power of writing generic code.