Validate attributes based on their types

[orhoj]

Purpose

Validate that add credential requests do not contain invalid attributes.

Changes

• Added validation when receiving a request to add a credential.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.