Add function for generating user invites

[rdimitrov]

Summary

The following PR adds a function for generating user invites.

It uses randomly generated 48 bytes which are then base64 encoded.

I wasn't sure if we should encrypt this but if you think it's necessary we can update it (best to happen in a follow up so we don't block other efforts)

Fixes #(related issue)

Change Type

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• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

Outline how the changes were tested, including steps to reproduce and any relevant configurations.
Attach screenshots if helpful.

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[coveralls]

coverage: 53.467% (-0.009%) from 53.476%
when pulling 1ccadb0 on invite-code
into 07f93e0 on main.