Remove any terminating / from the Credential Issuer Identifier when forming the Credential Issuer Metadata URL

[paulbastian]

Closes #744

My observations while building this PR:

• the actual cleanest approach to this would be to forbid trailing slashes for the Credential Issuer Identifier in the first place
  • however, this is more of a breaking change and not aligned with RFC8414, which likely must have overseen this more elegant solution, as it prevents this edge case at the AS/RS side, instead of at client parsing
  • we still could add this as a recommendation, if there is appetite
• RFC8414 is also missing an example for the trailing slash, which in my opinion is important to highlight, so I added another example into the text that actually showcases the rule that is re-added in this PR