Adding support for case insensitive search in unique and exists rules

• fix(validator): use prepared statements with unique and exists rules f10656b
• test: fix breaking tests 4dc1c29
• docs(README): add audit badge and update urls 417ac9d
• improvement: allow case-insensitive unique and exists queries 29f7dfc
• test: write tests for the provider 7bc9b7a

v8.3.0...v8.3.1