Add GetSenderInfo function to get message sender info

[risboo6909]

So I missed the feature to get an email address of the user sending message to my bot.

Opening PR, because this feature may be helpful for other people.

Usage pattern is like this (I omitted errors handling for simplicity):

act, _ := s.bot.ParseRequest(ctx, req)
senderInfo, _ := s.bot.GetSenderInfo(ctx, act)

senderInfo will contain user info (including email address, id, name, etc).