This commit allows to share spreadsheets to other users (public, portal or
internal without the required access rights)

The challenges of sharing odoo spreadsheets
-------------------------------------------

Odoo spreadsheets can have any data from the database.
ODOO.PIVOT and ODOO.LIST functions specifically can target *any model*
and *any field*.
The values are dynamically loaded with RPC calls when the
spreadsheet is open. Normal access rights apply to load this kind of
"embeded" data.

A user can open a spreadsheet if he can read the `documents.document` record,
but odoo specific functions might result in errors if the user doesn't have
the access rights on the underlying model.

That's obviously not what we want when sharing a spreadsheet to an external
person. We want this person to see the values and not a spreadsheet full
of errors.

Giving access to external user?
---------------------------------

Users must have a very clear understanding what they are "leaking" when
they share a spreadsheet. Sharing a spreadsheet should not open any door
the user wouldn't think of or wouldn't understand.

The best way is to be very strict with the data we are sharing.
That means: only the specific models, specific fields and specific records
visible in the spreadsheet by the user who is sharing (different users can
see different values for the same spreadsheet, depending on their access rights).

We also want to consider the following scenario:
Alice is a newcomer (with very limited access rights) and she shares a
spreadsheet to a customer. A few years later, she is manager and has a
lot more access rights (groups, ir.rules, etc.).
The forgotten spreadsheet shared years ago should not leak more data because
Alice now has access to all company data.

Specification
=============

With all those challenges in mind, here is a first approach of shared
spreadsheet:

Readonly freezed spreadsheet for external users
-----------------------------------------------

When sharing a spreadsheet, we actually copy and freeze the spreadsheet at that
time. Odoo formulas are replaced with their value. This is the easiest and safest
way to deal with access rights to other models: there's no access to other
models at all ^^
The spreadsheet is displayed in readonly since it would only be editing a copy.
If the external person wants data to be updated, he can ask a new sharing link.

Read/Write for internal users
-----------------------------

The situation for internal users is different. We can rely on their actual access
rights.
When an internal user opens a spreadsheet sharing link, he is redirected to the
regular spreadsheet client action. A token is used to read/write the
`documents.document` record (and other linked models such as `spreadsheet.revision`),
but the data for pivots, lists, etc. is loaded with the user's own access rights.
If the user doesn't have the rights to read a model or field, the function results in
an error and that's the expected behavior.
This sharing strategy is perfectly fine for all spreadsheets that doesn't contain
any odoo data (think of all the Google Sheets we receive internally by email to
register to an event or any other stuff).

Future work
-----------
From a functional point of view, the spec is far from perfect.
Users would probably expect the data to "update" itself (not freezed).
People will want write access for external users as well.
Given the complexity of getting it right (from a tecnical, security and functional POV),
this is left for a later work

Technical notes
---------------
The excel file is saved along with the freezed spreadsheet. We could export the
spreadsheet as excel, on demand, from the portal (as save a bit of storage)
but it would require to make the `/spreadsheet/xlsx` route public to zip the
generated files to the excel file.
And I don't want to do that.

Changes in the generic `spreadsheet_edition` module allows to easily implement
sharing for other business objects, such as spreadsheet dashboards.

Task: 3045808

community https://github.com/odoo/odoo/pull/114040