chore: use separate struct for Invite roles

[yokkkoso]

Use separate struct for Invite roles because as docs says:

This is a partial role object that only contains id, name, position, color, colors, icon, and unicode_emoji.

And currently, using the default Role sturct results in a unmarshalling error when performing a GET /invites/INVITE_CODE request:

ERROR error unmarshalling response body name=bot name=rest_client err="strconv.ParseInt: parsing \"\": invalid syntax" endpoint=/invites/INVITE_CODE

[topi314]

looks good, can you also add the same base methods as the normal role struct?

• String
• Mention
• IconURL