This commit introduces *external_tax* modules to gather common code in
account_avatax* and l10n_br_avatax*. When l10n_br_avatax* was
developed some of the code in account_avatax was duplicated.

Concretely this module contains:

- triggers that cause a tax computation, apart from manual calculation
taxes will be calculated in situations where an end customer is
viewing a record (e.g. a quote sent via email) and/or has the
opportunity to pay (e.g. /my/quote)

- code to set tax amounts on records that support tax
calculation (sale.order and account.move at the moment)

- filters that help integrations determine on which records tax should
be calculated

Tax integration modules can now be focused on building and processing
requests to external tax calculation services, and don’t need to be
concerned with these low-level mechanisms anymore.

In order to implement a new service the account.external.tax.mixin can
be inherited on models that need tax calculation (typically sale.order
and account.move).

- _compute_is_tax_computed_externally() should be overridden to return
True when appropriate

- _get_external_taxes() will be called when appropriate and is the
main entry point that should launch the external tax calculation
process

- _get_line_data_for_external_taxes() should be implemented to return
data needed for tax calculation for that particular model

- _get_date_for_external_taxes() should be overridden to return the
date on which tax calculation should be based

Other optional hooks are provided for uncommitting and voiding.

Upgrade PR: odoo/upgrade#5151