fix: use input for timestamp if datetime

[AstreaTSS]

About

This PR allows for timestamp in Embed to use a datetime if it's provided one. This also modifies convert_type to make this possible, allowing the use of classmethods, but that's more of a happy accident with how this fix worked out.

Checklist

• I've ran pre-commit to format and lint the change(s) made.
• I've checked to make sure the change(s) work on 3.8.6 and higher.
• This fixes/solves an Issue.
  • (If existent):
• I've made this pull request for/as: (check all that apply)
  • Documentation
  • Breaking change
  • New feature/enhancement
  • Bugfix