Add proto validation for invite codes

[rdimitrov]

Summary

The following PR adds proto validation for invite codes which are 48 random bytes that are then URL safe base64 encoded which end up being 64 bytes.

Ref https://github.com/stacklok/minder-stories/issues/94

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.
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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]

Pull Request Test Coverage Report for Build 11708002683

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.003%) to 54.766%

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Totals
Change from base Build 11707315932:	0.003%
Covered Lines:	15535
Relevant Lines:	28366

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💛 - Coveralls