Lib does not account for legacy bots/apps

[Kowlin]

Hey there, when trying this lib out on an old bot (3 years+) I've noticed that the bot is unable to do anything due the fact that the bot's User ID is not the same as its Application ID. So setting the user ID here sadly doesn't work for old bots.
https://github.com/eunwoo1104/discord-py-slash-command/blob/bbec2e0e109a8bcbd3d47206fb8651299e094d3b/discord_slash/http.py#L75

[eunwoo1104]

Unfortunately, this won't be fixed due to:

• We can't call discord.Client.application_info() before run() function is called and discord.py doesn't provide any cached/requested AppInfo instance, which makes implementing this issue's fix complicated.
• We can add app_id option at SlashCommand, but implementing that will be another major changes due to discord.Client.user doesn't exist before run() function is called, plus possible breaking change(s).

I could fix this issue, but still it will require major changes just to support old applications.