fix(brotli): Preserve `ETag` and `Last-Modified` headers in Brotli-compressed response

[joshdcu]

Description

Preserve ETag and Last-Modified headers in Brotli-compressed response.

Which issue(s) this PR fixes:

Fixes #12707

Checklist

• [x] I have explained the need for this PR and the problem it solves
• [x] I have explained the changes or the new features added to this PR
• [ ] I have added tests corresponding to this change
• [ ] I have updated the documentation to reflect this change
• [ ] I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[Baoyuantop]

Hi @yuweizzz, could you please confirm this change?

[yuweizzz]

It is ok to preserve those headers, but for the etag header, if it exists, you need to transfer it to weak etag.

and add a test case.

[Baoyuantop]

It is ok to preserve those headers, but for the etag header, if it exists, you need to transfer it to weak etag.

and add a test case.

Hi @joshdcu, can you make further modifications?

[joshdcu]

Yes, will be working on it soon, thanks for the reminder!

[Baoyuantop]

Hi @joshdcu, any update?

[joshdcu]

Sorry @Baoyuantop! I've been unavailable and haven't made a start yet. I would like to work on this eventually but I'm happy to let someone take this up if they want to.