fix(python,asyncio): multipart form data serialization (#19302)

* fix: object serialization for multipart requests

This PR is essentially
<https://github.com/OpenAPITools/openapi-generator/pull/18140> but for
the asyncio client.

* fix: int serialization for multipart requests

urllib3 handles serializing ints in post params (ref 1), while asyncio
explicitly does not (ref 2).

ref 1: <9316764e90/src/urllib3/filepost.py (L75-L76)>
ref 2: <https://github.com/aio-libs/aiohttp/issues/920>

* test: new fake multipart endpoint with files and body

* test: regression test for stringified body params

* fix: mypy tweak

* fix: FILES regeneration

* feat: object, int serialization for multipart reqs

Extends previous commits (and #18140) to cover the python-pydantic-v1
client as well.

* fix: use async with in test

* test: regression test for pydantic-v1-aiohttp

* test: add regression test to pydantic-v1

Also brings the second test in line with the first, patching
`urllib3.PoolManager.urlopen`

samples/client/echo_api/python-pydantic-v1/openapi_client/rest.py

@@ -201,6 +201,8 @@ class RESTClientObject:
                     # Content-Type which generated by urllib3 will be
                     # overwritten.
                     del headers['Content-Type']
+                    # Ensures that dict objects are serialized
+                    post_params = [(a, json.dumps(b)) if isinstance(b, dict) else (a,b) for a, b in post_params]
                     r = self.pool_manager.request(
                         method, url,
                         fields=post_params,