feat(Webhook): backport missing properties

[SpaceEEC]

Please describe the changes this PR makes and why it should be merged:

This PR backports the following properties of Webhook:

• avatarURL as a getter from feat(Webhook): addition of Webhook#avatarURL function  #3625
• type, createdAt, and createdTimestamp from feat(Webhook): add type property and created* getters #3585
• url from feat(Webhook): add url getter #3178

Status

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.