Fix cog slash commands when using ctx.invoke()

[DaTurr3t]

Summary

This PR fixes two simple things:

1. Application Commands in Cogs not having their cog set (see this Issue).
2. Checking if an Application Command is in a cog and calling its callback accordingly.

This means you can use ctx.invoke() for slash commands again like so:

class TestCog(commands.Cog):
    def __init__(self, bot: discord.Bot) -> None:
        self.bot = bot

    @slash_command(guild_ids=gids)
    async def other_command(self, ctx, greet) -> None:
        await ctx.respond(f"{greet} I should run now!")

    @slash_command(guild_ids=gids)
    async def invoke_test(self, ctx) -> None:
        await ctx.invoke(self.other_command, "Hi!")

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why
• 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, typehinting, examples, ...)

[krittick]

LGTM, tested with cogs and regression tested without cogs. ApplicationContext.invoke now works properly for both situations.