Added flag for lowercase email casting in GitHub connector

[thomwatkin]

Signed-off-by: Thom Watkin twatkin@provectus.com

Overview

Adds functionality from #1888 to the GitHub provider.

Follows for similar reasons - we don't always know what the casing of a given user's email will be, especially given that most email clients treat the value as case-insensitive.

What this PR does / why we need it

Closes #2578

Adds the ability for us to cast all user emails returned from GitHub to lowercase using the flag emailToLowercase

Special notes for your reviewer

N/A

Does this PR introduce a user-facing change?

Allows admins to optionally enable the `emailToLowercase` flag, which will cast all emails returned from Dex to lowercase. 

Disabled by default. 

Useful when you're unable to verify or match the casing of a user's email. 

[ms443]

LGTM