Add sparse enum decoding benchmarks

[osa1]

With upcoming change we'll improve decoding performance of enums, but there will be a difference between "sparse" and "dense" enum decoding performance, even though they'll both be faster.

To be able to measure the difference add a "sparse" enum type and a benchmark for decoding it.

"Sparse" means the enum has large gaps between known enum values, or negative enum values.

When decoding this kind of enums, the mapping from the wire varint to the Dart value for the enum needs to be done by binary search, map lookup, or similar.

For "dense" enums, we can have a list of enum values and index the list directly with the varint value, after a range check.

These changes will be done in the follow-up PR(s).