feat: VoiceClient.play wait_finish parameter for awaiting end of stream

[davidhozic]

Summary

Added wait_finishparameter to VoiceClient.play, which will return an awaitable if True, that can be used to wait for the audio to stop playing. If set to False (default), the function will not block and None will be returned

Example:

import discord
import secret

client = discord.Client()

@client.listen("on_ready")
async def start():
    guild = client.get_guild(863071397207212052)

    channel: discord.VoiceChannel = guild.get_channel(1132373179210399844)

    def callback(exc):
        pass

    voice_client = await channel.connect()
    res = await voice_client.play(
        discord.PCMVolumeTransformer(discord.FFmpegPCMAudio("VoiceMessage.mp3"), 0.25),
        after=callback,
        wait_finish=True
    )
    print(f"Set exception: {res}")

    voice_client.play(
        discord.PCMVolumeTransformer(discord.FFmpegPCMAudio("VoiceMessage.mp3"), 0.25),
        after=callback,
        wait_finish=False
     )

client.run(secret.TOKEN)

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.

[Middledot]

Tested, could be useful