Better error message when buckets_path refers to multi-bucket agg

[polyfractal]

Instead of throwing a technical error when the user specifies a pipeline agg path that includes a multi-bucket, we can check during the property resolution and throw a friendlier exception

The old exception:

buckets_path must reference either a number value or a single value numeric metric aggregation, got: java.lang.Object[]

Compared to the new exceptions:

[histo2] is a [date_histogram], but only number value or a single value numeric metric aggregation are allowed.

[the_percentiles] is a [tdigest_percentiles] which contains multiple values, but only number value or a single value numeric metric aggregation are allowed. Please specify which value to return.

I'm not super pleased with how this was implemented, adding a boolean to getProperty(). But it seemed the least invasive way. In particular because we have to deal with both multi-buckets and numeric aggs that have multiple values.

Note: this uses the same changes to AggregatorTestCase as #29641 so we can test pipelines easier. Not relevant anymore

Closes #25273

[elasticmachine]

Pinging @elastic/es-search-aggs

[colings86]

@polyfractal I assume this is still something we want to get merged? Should we update the branch and find a reviewer?

[polyfractal]

Urgh yes, forgot about this. Will get it updated and find a victim reviewer :)

[polyfractal]

Hmm, I'm looking at this again and I don't really like how it was implemented. I'll carve out some time to re-examine the issue and see if it can be fixed a different way that doesn't need to change the method signatures. Just feels like a lot of complication for a nicer error message.

[polyfractal]

Closing for now. I've decided I don't like the implementation, and don't think there's a need to keep this PR pending/stalled while (or if) I think about a better way.