Allow users to add approval rules on buttons through Studio.

The rationale is that a lot of SMEs want to establish an approval
process where doing an action on a record (typically through the use of
a button) is limited to certain users (or must be approved by certain
users). This commit add such a mechanism through the Studio application.

The approval rules are stored through 2 models:
- a `studio.approval.rule` model, which represents a single approval
  rule tied to a model and an action or method (e.g. `sale.order`/`action_confirm`
  or `sale.order`/298 where 298 is, say, the id of the window action of a wizard).
  An approval rule is linked to a group and a user can only it if they are a member
  of that group.
- a `studio.approval.entry` model, which represents the approval (or
  rejection) of a rule for a specific record by a specific user.

The main entry point for these models are:
- `get_approval_spec` on `studio.approval.rule`, which returns a full
  description of the approval flow for a given action (and if the spec
is requested for a specific record, also includes existing entries).
- `check_approval` on `studio.approval.rule`, which checks whether
  the user can proceed with an action (method or action ID for a specific
  record); if some rules for the action have not yet been  they will be
  automatically approved by the current user on the fly if they are
  allowed to approve them.
- `set_approval` on `studio.approval.rule`, which records an entry for
  that rule (accepted or rejected) for a specific record.
- `cancel_approval` on `studio.approval.rule`, which deletes an entry for
  that rule (accepted or rejected) for a specific record.

On the client's side, several overrides have been added so that any
click on a button which has been marked as being subject to approval
through a `studio_approval` attribute on its node will first check
server-side if the action can proceed through a call to
`check_approval`; if the check is successful, the action proceeds; if
not, it is stopped and a warning is displayed to the user.

This commit also adds the necessary plumbing in `web_studio` to allow
configuring these approval rules on any button of a form view.

Current limitations of this implementation:
- only buttons in form views support this mechanism
- these checks are done through the web client only; a badly-intentioned
  user may bypass these checks through an rpc calls using their own
  credentials

This is a simple feature made for quick and easy rules; any advanced use
case should instead use automated actions based on complex rules - these
will not be by-passable through rpc calls. But, for small companies that
need a quick and easy way to restrict actions, this is a nice addition
to Studio.

Task-2092572

Community changes in odoo/odoo#48983