[ADD] fields.py: New x2many command helper

We provide a new helper class to help redact x2many commands for create
and write methods. To ensure best compatibility with the xmlrpc layer we
do not change the protocole, the commands are still 3-elements tuples
where the first element is still an integer in between 0 and 6. The
helper class provide the cannonical constants and static methods to ease
working with the commands. The new helper class is also available in QWeb.

Developers are encouraged to transition their code so it uses this new
helper class.

Task: 2366606