Add parsing for InternalGeoCentroid

[cbuescher]

This adds parsing to the InternalGeoCentroid aggregation. One problem I encountered here is that we don't render the count parameter that is available through the count() method in the GeoCentroid interface to REST (see #24366). After discussing this it looks like we should add this parameter to the REST output of the aggregation. I did this in this PR already but I think I will also open a separate PR to do this in master already.
I'm not sure we can do anything in 5.x if we backport, we will need to work with a dummy constant for the count value there since it is not available via REST.

[javanna]

LGTM

[javanna]

out of curiosity: do we have similar shared methods for toXContent in some other place? not a huge deal but I am a bit on the fence on adding these. Ideally the two impls wouldn't depend on each other and one day will be split apart. On the other hand we can ignore this and fix it later when needed.

[cbuescher]

No, we don't. I tried this in one PR by sharing something similar in the interface but that isn't such a good solution either. In former cases the two toXContent were slightliy different, in this case they are really similar, thats why I didn't want to copy them. But I get the point about dependencies and can copy the toXContent over if you prefer.
One other note on this: we often share ParseFields or at least the field keys between InternalX and ParsedX, should we also already start pulling them out, e.g. into the interfaces? Maybe better later when we eventually do the splitting at some point in the future?

[javanna]

Good point about the parse fields. Let's not worry too much for now. We can't be consistent if we don't enforce these constraints. Let's share these simple things for now and one day we will change that when we need to.

[cbuescher]

@javanna thanks, I reverted the shared xContent rendering and will merge this to the feature branch with the added count field in InternalGeoCentroid. I will also open an issue that we merge this part to master individually.

[cbuescher]

@javanna I opened #24387 to add the 'count' parameter to 'geo_centroid' on master already. It should be similar to 49c36da. I can either merge this with our feature branch and we get #24387 in later via merge with master or we can leave this open until the PR against master is merged.

[javanna]

@cbuescher your call I am fine either way.