Add varchar/char cast pushdown support in Redshift

[mayankvadariya]

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Description

This PR adds support for cast projection pushdown for the Redshift connector for char/varchar types.

• char -> char
• varchar -> varchar

Additionally, enforces case-sensitive comparison by using Redshift COLLATE function https://docs.aws.amazon.com/redshift/latest/dg/r_COLLATE.html

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# Section
* Add varchar/char cast pushdown support in Redshift. ({issue}`issuenumber`)