Minor: make it clear Predicate is crate private

[alamb]

Which issue does this PR close?

• Follow on to Support Binary arrays in starts_with, ends_with and contains  #6926

Rationale for this change

Predicate is marked pub but it is in a non public module

arrow-rs/arrow-string/src/lib.rs

Line 24 in 595a835

mod predicate;

This gives it a false impression that it is part of the public API, which it is not:

• https://docs.rs/arrow/latest/arrow/index.html?search=Predicate
• https://docs.rs/arrow-string/latest/arrow_string/?search=Predicate

What changes are included in this PR?

Mark pub(crate) to make it explicit that this structure is not public outside the crate

Are there any user-facing changes?

[alamb]

Thank you @tustvold