feat: handle existing usernames and emails during registration (AAI-518)

[minh-biocommons]

Description

AAI-518: Handle existing usernames and emails during registration

Changes

• Updated BioCommons & SBP registration endpoints to return more specific email/username exist error message along with field errors instead of just a generic "both exists"
• Added utils endpoints for checking existing username & email to be used during registration form step validation
• Added/updated relevant tests

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have added unit / integration tests that prove my fix is effective or that my feature works
• I have run all tests locally and they pass
• I have updated the documentation (if applicable)
• For any new secrets, I have updated the shared spreadsheet and the GitHub Secrets.

[marius-mather]

Looks good but I think we need to do more detailed checking for existing usernames - the Auth0 API lets you search by username but may not return exact matches

[marius-mather]

Looks good now, thanks!