Add a ext_authz fuzzer for the http client

[antoniovleonti]

Commit Message: Add a ext_authz fuzzer for the http client
Additional Description: This PR creates a new fuzzer under the ext_authz directory that uses the ext_authz http client. It moves shared code between the new http fuzzer and the existing grpc fuzzer into a fuzzer utils library.
I also "smuggled in" some tiny improvements to the ext_authz code where I found some redundant copies of shared_ptr variables.
Risk Level: n/a
Testing: test-only PR
Docs Changes: none
Release Notes: none
Platform Specific Features: none
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[Optional Fixes #Issue]
[Optional Fixes commit #PR or SHA]
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[antoniovleonti]

Half the commits in this PR are already merged into main. This is because I started working on this PR before #32859 was finished. I tried to remove them, but I don't know how nor do I know if it matters.

[antoniovleonti]

/retest

[antoniovleonti]

/retest

[htuch]

@yanjunxiang-google can you take a first pass? @ggreenway can provide senior maintainer coverage and owner. CC @tyxia

[yanjunxiang-google]

Half the commits in this PR are already merged into main. This is because I started working on this PR before #32859 was finished. I tried to remove them, but I don't know how nor do I know if it matters.

It's hard to review a PR with changes already merged into main. Maybe you can create a fresh git workspace and a fresh child branch, and just have your new changes there then send out a PR for it? That will help us to understand what's changed.

[antoniovleonti]

Attempting to clean up commit history with another PR as Yanjun said.