* [opentelemetry-cpp] unvendor absl polyfills
By default opentelemetry vendors-in a copy of some Abseil polyfills
(e.g. `absl::variant`). That makes it hard (I would claim nearly
impossible) to use both the abseil and the opentelemetry-cpp ports at
the same time, as both define `absl::variant` in some of their headers.
It seems better to always use the `-DHAVE_ABSEIL=ON` CMake option and
depend on the abseil port. Outside vcpkg it may be useful to use such
vendored-in copies of the code, as that "simplifies" dependency
management. In the context of vcpkg that is just wasteful.
Simplified how to support `abseil[cxx17]` by removing some code in
opentelemetry that forcibly downgraded the compiler to use C++11. Add a
missing dependency for gRPC generated code. This code indirectly depends
on Abseil too.
* Opentelemetry is missing a find_dependency() when -DHAVE_ABSEIL=ON
