Benchmarks: explicit group

When declaring a benchmark, the local names of its models are used to group benchmarks together during the inference stage of a full Tezt run.

However, benchmarks are being changed to have exactly one model, so this calls for a simplification and a clarification: let's make this grouping explicit.

• Add a group optional value to all benchmarks: !8541 (merged).
  • It should be set to Some group_name for benchmarks that need to be grouped together, and None for others.
  • #4691 (closed) makes some propositions about what benchmarks could be grouped together.
• Add a group field of type string option to src/proto_alpha/lib_benchmarks_proto/Benchmarks_proto.Benchmark.S.
  • At registration time, the local name of the model should be set to group_name if group = Some group_name, and to an arbitrary name otherwise, i.e. when group = None (for instance the benchmark name suffixed by /model, which is already the case with Benchmarks_proto.Registration.register).