Add get_my_volume method

[New-dev0]

Ik, it can be called as Telethon/Pyrogram method, but will be glad to have that in tgcalls as friendly method...

[MarshalX]

You should create more abstract method for getting participants in mtproto bridges. Don't hardcore limit, peers and another args of mtproto method. You can pass it from more high lvl. From the GroupCallBase, for example. Also it is scared to see many if statements in mtproto bridges