Add an option handling UTF-8 decode errors in certain SSH packets

The SSH RFCs define that strings in some SSH packets must be encoded
as UTF-8, and AsyncSSH enforces this by raising a ProtocolError when
UTF-8 decoding fails. This commit adds a new config option to specify
an error handling strategy for the following cases:

  * Disconnect reason
  * Debug message
  * Userauth banner
  * Channel open failure reason
  * Channel exit signal reason
  * SFTP error reason

The default is 'strict', whch preserves the existing behavior of
raising a ProtocolError, but selecting other options allows the
invalid bytes to be removed or replaced, avoiding the exception.

Thanks go to GitHub user Le-Syl21 for suggesting this!