Minor: Extend more style of udaf expr_fn, Remove order args forcovar_samp and covar_pop

[jayzhan211]

Which issue does this PR close?

Closes #.

Part of #10484

Rationale for this change

Improve expr_fn usage

1. add expr_fn for count(arg, dictinct) where has additional argument distinct
2. covariance_pop(arg) where has no any additional argument

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

[alamb]

This change now means the expr_fn can't be used with ordering / null_treatment right? I think that is probably ok

Perhaps we can consider some better API for creating aggregate functions with ORDER BY, perhaps inspired by #6747

Perhaps something like

// form `FIRST_VALUE(a ORDER BY b)`let agg_expr = AggregateUDF::call(first_value()).args(col("a")).order_by(vec![col("b")).build()?;

[jayzhan211]

This change now means the expr_fn can't be used with ordering / null_treatment right? I think that is probably ok

Perhaps we can consider some better API for creating aggregate functions with ORDER BY, perhaps inspired by #6747

Perhaps something like

// form `FIRST_VALUE(a ORDER BY b)`let agg_expr = AggregateUDF::call(first_value()).args(col("a")).order_by(vec![col("b")).build()?;

This looks nice