[FIX] A more DeveloperFriendly Method for Extending the Generation of Journal Entries from account.model

Currently, it is not possible to extend account.model.line and propagating those changes to account.move.line because "account.model".genarate() method is not as DevFriendly, in order
to leverage from this model, you have to override the whole method and create a new one,
What I intend later is to just _super_ a new method called generate_line_values that can be modify by
any one eager to extend account.model.line, e.g., adding new fields as Analytic Distribution,
As Analytic Distribution lacks of this behaviour by default.
@odony & @qdp-odoo Expect you could weight this PR, I know it could be hard but let us try.

Best Regards.
This is because if someone else changes the context, this changes aren't passed to the main context, and this main context does not get contaminated, and this way unaware changes will not be expected as the ctx2 is only contained in the def generate_line_values