ES|QL: Set FUSE to execute on coordinator

[ioanatia]

Tracked in #123389

FUSE supports RRF and linear combination as the two method to assign new relevance scores.
FUSE cannot be pushed down to the data nodes, it needs to see the whole result set in order to assign new scores.

Because of this we need to:

1. Ensure that FUSE can only be executed on the coordinator: we add the ExecutesOn.Coordinator and PipelineBreaker to FuseScoreEval.
2. We want to make sure the input of FUSE is always limited - this makes sense in practice, since using RRF or linear combination is used with limited results sets. We don't use RRF or score normalization on results set of millions of docs.
   What this means practically is that the we are looking for another PipelineBreaker exists before FUSE.

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[carlosdelest]

This is nice. 👍

TIL about PipelineBreaker and ExecutesOn.

[carlosdelest]

Nit - I understand why this is needed, but probably makes no sense in practice. Maybe we can check that there's a ROW at the top and avoid the need for LIMIT in that case, as input will be already limited?

[ioanatia]

I guess the usage of ROW is limited in practice, especially in combination with FUSE, but I added a note in #123389 and will definitely look into this as a follow up.