Added strict validation for username and password in backend.

[aryan-bhokare]

Proposed changes

Added validation for username and password in backend.
Following #4650

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• New feature (non-breaking change which adds functionality)
• Bugfix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices applies)

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Dependency

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Special notes for your reviewer:

[SarthakJain26]

@aryan-bhokare, can you please add an ENV to enable/disable the strict password validation. This ENV (ENABLE_STRICT_PASSWORD_POLICY) will be true by default, i.e. strict password policy check will be implemented, but user can toggle that off based on the use case.