[7.5][ML] Deduplicate multi-fields for data frame analytics (#48799) #48807
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In the case multi-fields exist in the source index, we pick
all variants of them in our extracted fields detection for
data frame analytics. This means we may have multiple instances
of the same feature. The worse consequence of this is when the
dependent variable (for regression or classification) is also
duplicated which means we train a model on the dependent variable
itself.
Now that #48770 is merged, this commit is adding logic to
only select one variant of multi-fields.
Closes #48756
Backport #48799