replace fmt::format with abseil

[derekargueta]

Abseil has all the string processing functionality we need, this would allow us to remove a dependency.

[mattklein123]

@derekargueta I would discuss with @jpeach who was also looking into this. FWIW unless we have a plan to replace it also in logging (it is baked into spdlog pretty heavily) I don't see a huge amount of value in the churn here. If we can replace it everywhere that would be nice.

[derekargueta]

Yeah the idea for this was actually from a slack message @jpeach posted a while ago in #envoy-dev.

I was able to knock out a lot of easy conversions from fmt::format("some string: {}", foo); to absl::StrCat("some string: ", foo); with a regex transform from:
fmt::format\("(([A-z]|\s)*)\{\}",\s(([A-z]|\.|_|\(|\))*)\);
to
absl::StrCat("$1", $3);
But for the more involved format templates we'll definitely need something a little more complex.

[jpeach]

There's clang tooling to do this here: https://github.com/jpeach/rewrite-fmt-format.
Previous discussion in #8963

There are a very few places (maybe 5) that use fmt formatting specifiers other than "{}". These need to be done by hand. There are also a couple of cases where the format string isn't a literal, which also need to be done by hand.

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[htuch]

Given that fmt is now in C++20, I think we should stick with fmt for the long term FWIW (CC @ahedberg)

[ahedberg]

It is a little weird to have two formatting libraries, but probably not worth the churn. Maybe some rule like "use fmt in logging, use absl everywhere else" suffices.

If the ultimate goal is to remove dependencies, Abseil is releasing its own logging framework soonish, which could let us cut spdlog and fmt if we wanted.