Implement Account.UpdateUsername

[aykutalparslan]

Changes username for the current user.

userEmpty#d3bc4b7a id:long = User;
user#3ff6ecb0 flags:# self:flags.10?true contact:flags.11?true mutual_contact:flags.12?true deleted:flags.13?true bot:flags.14?true bot_chat_history:flags.15?true bot_nochats:flags.16?true verified:flags.17?true restricted:flags.18?true min:flags.20?true bot_inline_geo:flags.21?true support:flags.23?true scam:flags.24?true apply_min_photo:flags.25?true fake:flags.26?true id:long access_hash:flags.0?long first_name:flags.1?string last_name:flags.2?string username:flags.3?string phone:flags.4?string photo:flags.5?UserProfilePhoto status:flags.6?UserStatus bot_info_version:flags.14?int restriction_reason:flags.18?Vector<RestrictionReason> bot_inline_placeholder:flags.19?string lang_code:flags.22?string = User;
---functions---
account.updateUsername#3e0bdd7c username:string = User;