Refactor percentile_cont to clarify support input types (#19611)

## Which issue does this PR close?

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- Related to #18092

## Rationale for this change

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Current signature for `percentile_cont` is quite confusing in what types
it accepts. Currently the code suggests it accepts all numeric types,
where floats & decimals are maintained as is, integers are cast to
float64 internally. However this is misleading as `NUMERICS` does not
contain decimals, so currently everything is cast to float.

Clean up the code to make this more clear and do various other
refactors.

## What changes are included in this PR?

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Use coercion signature to have type coercion coerce types to float so we
can remove internal code related to casting to float or handling
non-float types.

Various other refactors.

## Are these changes tested?

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Existing tests.

## Are there any user-facing changes?

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No.

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Author: Jefffrey