Add username from env mysql scaler

[Indresh2410]

Provide a description of what has been changed

Checklist

• When introducing a new scaler, I agree with the scaling governance policy
• I have verified that my change is according to the deprecations & breaking changes policy
• Tests have been added
• Changelog has been updated and is aligned with our changelog requirements
• A PR is opened to update our Helm chart (repo) (if applicable, ie. when deployment manifests are modified)
• A PR is opened to update the documentation on (repo) (if applicable)
• Commits are signed with Developer Certificate of Origin (DCO - learn more)

Fixes #5883

Relates to #5883

Docs PR: kedacore/keda-docs#1419

[SpiritZhou]

Could you mind to refactor this mysql scaler to new declarative scaler config (#5797) ? Then we don't need to add a new function GetFromAuthOrMetaOrEnv.

[Indresh2410]

Could you mind to refactor this mysql scaler to new declarative scaler config (#5797) ? Then we don't need to add a new function GetFromAuthOrMetaOrEnv.

Thanks @SpiritZhou . Have addressed it