`@keystone-6/auth` identity field should not be case sensitive

# Bug report
## Describe the bug

Not sure if this is exactly bug or some configuration is needed.

I noticed that authentication does not ignore case when matching `identityField`.

## To Reproduce

1. Run auth example
2. crate user with first letter of email capitalize
3. try signing in using all small letters in email using graphql playground

## Expected behaviour

authentication should succeed

## Screenshots

If applicable, add screenshots to help explain your problem.

## System information

NA

Start with the @keystone-6/auth authentication flow in the auth example and reproduce the identityField case mismatch through GraphQL Playground. Trace how the identityField is matched, then verify that a user created with a capitalized email can sign in using lowercase input.

Written by the indexing model from the issue text.