This refactor is based on the idea that CI for beman repositories
should only contain configuration, and the logic powering the CI
should live in reusable workflow files that can be deduplicated using
infra (the move to infra will happen in a follow-on commit).

After this commit, CI for exemplar is configured via a JSON file whose
contents are processed by the new reusable-beman-build-and-test.yml
workflow file.

For the GCC and Clang builds, the configuration dispatches to the new
containers provided by bemanproject/infra-containers. Otherwise, it
uses GitHub runners.

The configuration chosen for exemplar is intended to provide a sane
default for other projects. Most configurations only run a default
build, in release mode so that the tests execute quickly. Debug and
sanitizer builds are performed on the latest compiler version and C++
standard for each compiler, to ensure the most robust results.

Although a reflection build is not provided here, I've tested it using
the following configuration:

"clang-p2996": [
  { "versions": ["trunk"],
    "tests": [
      { "cxxversions": ["c++26"],
        "tests": [{"stdlibs": ["libc++"], "tests": ["Release.-DCMAKE_CXX_FLAGS='-freflection-latest'"]}]
      }
    ]
  }
],

This uses the ability to provide custom parameters to CMake after the
'.' separator to provide the reflection flag.