Note that this PR includes splitting the sampler aggregation into "sampler" and "diversified_sampler" aggregations for the API to make validation and parsing easier

I actually like the split. I'm wondering that we might rename sampler to eg. quality_sampler to make it clearer how it works (as opposed to eg. a random sampler?).

@jpountz I pushed a commit with your suggestions implemented. I'm not a fan of quality_sampler as the diversified_sampler can also be thought of as sampling for quality purposes. Personally I think sampler is fine as its the simple no frills version of sampling
/cc @markharwood

LGTM