Add support for Entry Point Command

[Soheab]

Summary

This PR adds the relevant fields and types for the Entry Point Command.

Discussion thread: https://canary.discord.com/channels/336642139381301249/1302049817178013866

Relevant API Docs:

• Application Commands // Entry Point Commands

Classes:

• EntryPointCommandHandlerType (enum)
• _PrimaryEntryPointApplicationCommand (type @ discord/types/command.py)

Attributes:

• AppCommandType.primary_entry_point
• AppCommand.handler -> EntryPointCommandHandlerType | None

Methods:

• AppCommand.is_default_entry_point_command() -> bool
  I added this one.. it may be useful? I don't know.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)