Allow single percent in PercentilesAggregation

[pquentin]

I don't know if we actually want to do that, but that's possible.

First, percents is parsed in PercentilesAggregationBuilder but I don't understand where a single value is allowed

However there's a YAML test that uses this form, the response looks like this:

              "tdigest_percentiles#percentiles_agg": {
                "values": {
                  "50.0": null
                }
              },

However, since it's undocumented, I'm happy to ignore that test.

[github-actions]

Following you can find the validation changes against the target branch for the API.

API	Status	Request	Response
search	🔴	2593/2615 → 2594/2615	2615/2615

You can validate this API yourself by using the make validate target.

[flobernd]

Would be fine for me. The "single or many" construct shouldn't even be breaking for the statically typing clients - so backport might be possible.