chore: expose TypeCoercionRewriter::new and add examples for type coerce expressions

[niebayes]

Which issue does this PR close?

Closes #13375.

Rationale for this change

What changes are included in this PR?

• Expose TypeCoercionRewriter::new.
• Add examples for applying type coercion to expressions.
• Update out-of-date README for the datafusion-sql module

Are these changes tested?

Are there any user-facing changes?

The TypeCoercionRewriter::new is now made public.

[niebayes]

@alamb Thanks for reviewing so quickly. I have submitted the changes.