Fix parsing single rescore element in SearchSourceBuilder

[cbuescher]

We are currently only parsing the array-syntax for the rescore part in SearchSourceBuilder ("rescore" : [ {...}, {...} ]) .
We also used to support "rescore" : {...}, so adding this back to the parsing in SearchSourceBuilder.

Closes #18439

[cbuescher]

@MaineC can you take a look at this and try this change with the query you used in #18439? I added a simplified version to the json parsing tests that I think will also work for the issue you reported.

[MaineC]

LGTM - thanks for fixing

[nik9000]

Should SourceSoureBuilder#toXContent spit out just the rescore object if there is only a single rescore? That is what we expect users to do when they build a rescore.

[cbuescher]

@nik9000 I think the reason we didn't do this (and other similar things like writing default values even if they haven' been explicitely set by the user) so far is to avoid having different query rendering logic. In this case it would have helped with test coverage (no need for the explicit json tests added with this PR) but in the end the array syntax with one element is just parsed to the same object again. Given that toXContent should only be used in roundtriup tests and for logging I think we can leave that for now.