Add the ability to define a variable for task output (including structured output)

[tomlynchRNA]

• For free text, a single variable name field for the whole task output
• For structured output json, one variable name field per field

When a task completes, save the output or each structured output field to the variables on the session.

I included Charles for agent-backend help because its probably better to save these from the agent-backend side when theyre generated as opposed to trying to distinguish output messages inside the webapp socket handler.

ref #569

[iandjx]

@charl3sj I'll be adding a task property called taskOutputVariableName
intead of doing a regex search i.e.

if not task.isStructuredOutput:
        match = re.search(r'^{([\w_]+)}$', task.expected_output)
        if match:
            var_name = match.group(1)
            session_variables = session.variables.copy()
            session_variables[var_name] = str(task_output)

get the variable name from task.taskOuputVariableName and update the session variable with that key value.