[FIX] SAML login saves invalid username when receiving multiple values

[pierre-lehnen-rc]

Proposed changes

• Join all values into a single string for fields that absolutely need to be a string;
• Change the user map JSON parser to allow specific array elements to be picked instead of the whole array.

Issue(s)

When the Idp sends multiple username values, Rocket.Chat is saving all of them as an array.
Closes #14895

How to test or reproduce

Screenshots

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves a current function)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Hotfix (a major bugfix that has to be merged asap)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works (if applicable)
• I have added necessary documentation (if applicable)
• Any dependent changes have been merged and published in downstream modules

Changelog

Further comments