Add benchmark results dataclass

[alistaire47]

As eventually we want to move the posting of results out of this package, this package needs to be able to save results to JSON in the same fashion {arrowbench} does. There are a few steps to this project, of which this is the first:

1. The first step is to make a dataclass (pydantic or native) that aligns with the schema we're currently using.
2. Use of the new dataclass is beyond the scope of this issue; that will be the next story.
3. Actually writing the JSON can be delayed for now, provided the capability is there. This should be simple whenever we're ready.
4. Removing the current calls to conbench.record() (once we've got a separate tool to do so) will likely take a little more work, but allow us simplify the codebase a bit. This may not happen for a bit, but should be kept in mind during (2) especially so we can end up with a tidy codebase.

Again, this task is only (1); the rest above is just for context.