feat: add support for custom status

[Dorukyum]

Summary

Closes #2204.
Defaults Activity.name to "Custom Status" if the type is ActivityType.custom when initializing, but not sure if this is the best way to do it. This currently requires developers to explicitly set the type field to ActivityType.custom because it defaults to ActivityType.unknown, as seen in the example below:

await bot.change_presence(activity=discord.Activity(type=discord.ActivityType.custom, state="I'm Batman."))

I feel like this should be made easier on the developer frontend, resulting with the following:

await bot.change_presence(activity=discord.Activity(state="I'm Batman."))
# or maybe even this:
await bot.change_presence(activity=discord.Activity("I'm Batman."))

I'm ready to push the change necessary to implement the last suggestion if the idea is approved.

Information

• 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, ...).

Checklist

• I have searched the open pull requests for duplicates.
• 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.
• I have updated the changelog to include these changes.

[Dorukyum]

CustomActivity.state will now be set to .name by default, which means developers can now write the following code to create a custom bot status:

activity = discord.CustomActivity("I'm Batman.")

This will have the same effect as:

activity = discord.Activity(type=discord.ActivityType.custom, state="I'm Batman.")

[OmLanke]

LGTM