[FIX] base: no request.is_frontend attribute in RPC

Install website then render a qweb template via xmlrpc, attribute error:
`request` has no `is_frontend` attribute.

Inside the qweb's `_prepare_environment` override of the http_routing
module (a dependency of website) is the following code snippet:

    if (not irQweb.env.context.get('minimal_qcontext') and
            request and request.is_frontend):
        return irQweb._prepare_frontend_environment(values)

The conditionnal is about injecting extra informations in the qweb's
context in case we are serving a frontend request. By default, there is
no `is_frontend` attribute on the request object. That attribute is set
by the ir.http's `_match` override of the http_routing module (a website
dependency): it is set True when we don't match any endpoint or that we
match an endpoint that is `website=True`, it is set False otherwise.

The ir.http's `_match` method is called whenever we are serving a http
request whoose session is bound to a specific database. i.e. when there
is a valid database saved in the request's session. When there is not
database in the request's session (or that it is invalid) the matched
endpoint is directly called without going throught ir.http. Most
endpoints are only accessible via ir.http.

The two `/xmlrpc` and `/jsonrpc` endpoints are examples of endpoint that
do not require an established database connection to work. They perform
the request authentication and database connection themselves. It is
possible to call those two endpoints with no database saved in the
session, thus it is possible to call those two endpoints without going
throught ir.http. This is expected.

The two endpoints's duty is to execute public model methods and return
the xml/json serialized result. To do so, a registry is loaded on the
database with all the installed modules, including http_routing.

We fall in a situation where (1) there is a request, (2) we are using a
registry where http_routing is loaded, (3) there is no `is_frontend`
attribute on `request` as we didn't serve the endpoint via ir.http. This
situation is illegale.

To solve the problem, instead of working on the `if request.is_frontend`
bit of the above conditional, we decided to work on the `if request`
bit. ISO-model wise, RPC is an extra 8th layer built on top of HTTP.
HTTP is merely a transparent transport between a RPC client and a RPC
server, any other request-response capable procotol could fit. The
method executed via RPC must be independant from the usage of HTTP as
mean of transportation thus it should not be capable of using the
current request.

The proposed change is to temporary un-expose the current request from
the local-stack during the execution of the RPC method. This fixes the
problem as the code now run like it was executed from the shell or from
a cron. The other benefit is that the pattern used inside the condition:
`if request and request.is_frontend` doesn't need to change.

X-original-commit: f28863bfdaac3f644fcb274c856e0770783ed75c
Part-of: odoo/odoo#104567

odoo/http.py

@@ -975,6 +975,15 @@ class Session(collections.abc.MutableMapping):
 _request_stack = werkzeug.local.LocalStack()
 request = _request_stack()
 
+@contextlib.contextmanager
+def borrow_request():
+    """ Get the current request and unexpose it from the local stack. """
+    req = _request_stack.pop()
+    try:
+        yield req
+    finally:
+        _request_stack.push(req)
+
 
 class Response(werkzeug.wrappers.Response):
     """