Bring forward support for nested assumes expressions on 3x

[cderici]

Description

This forward ports the fix for nested assume expressions from #940 into the master branch.

Fixes #938 for 3.x.

QA Steps

Same QA steps can be followed from #940, so copy/pasting the manual QA steps below:

---

Unfortunately it's not trivial to write an integration test for this, as the code that's been changed is part of the facade api. So we'll use the mysql-k8s charm that's been used in #938.

 $ juju version
3.x
 $ juju bootstrap microk8s micro28
 $ juju add-model assumes-test

 $ python -m asyncio
asyncio REPL 3.10.12 (main, Jun 11 2023, 05:26:28) [GCC 11.4.0] on linux
Use "await" directly instead of "asyncio.run()".
Type "help", "copyright", "credits" or "license" for more information.
>>> import asyncio
>>> from juju import model;m=model.Model();await m.connect();await m.deploy("mysql-k8s", channel="8.0/edge", trust=True)
<Application entity_id="mysql-k8s">
>>>
exiting asyncio REPL...

Notes & Discussion

JUJU-4562

[anvial]

QA passed, code looks good.

[cderici]

/merge