cubicweb #2099892 Improve transaction_data API [deprecated]

If transaction data can't be used as a standard dictionnary on the web server side. There is an offical API on Session/Request to manipulated it. This API consist in two simple method allowing the following operation:

• set a value of a key
• get a value for a key
• pop a value for a key

Having a more powerful API would have been much nicer in several situation.

I suggest having an official API to handle nested dictionary in transaction data. Key for each dictionnary could be see as namespace. Once be have a namespace concept in transaction data, we can have an API to:

• set a value for a key in a namespace
• get a value for a key in a namespace
• pop a value for a key in a namespace
• setdefault a value for a key in a namespace
• clear a whole namespace
• update a namespace with another dictionnary.

Having a way to operate on set should be nice too.