This change:
* Removes the patched Conan recipes from the `external/` directory.
* Adds instructions for contributors how to obtain our patched recipes.
* Updates the Conan remote name and remote URL (the underlying package repository isn't changed).
* If the remote already exists, updates the URL instead of removing and re-adding.
* This is not done for the libXRPL job as it still uses Conan 1. This job will be switched to Conan 2 soon.
* Removes duplicate Conan remote CI pipeline steps.
* Overwrites the existing global.conf on MacOS and Windows machines, as those do not run CI pipelines in isolation but all share the same Conan installation; appending the same config over and over bloats the file.
* Copy Antithesis SDK version 0.4.0 to directory external/
* Add build option `voidstar` to enable instrumentation with Antithesis SDK
* Define instrumentation macros ASSERT and UNREACHABLE in terms of regular C assert
* Replace asserts with named ASSERT or UNREACHABLE
* Add UNREACHABLE to LogicError
* Document instrumentation macros in CONTRIBUTING.md
