TypedDicts in Python can be marked with total=None. This means that omitting a field still results in a valid structure.
It gets trickier when non-total TypedDicts have also nullable fields.

For instance, imagine the following structure:

class PyType(TypedDict, total=False):
    height: int | None
    weight: int | None

The resulting schema would be something like that:

{
  "type": "record",
  "name": "PyType",
  "fields": [
    {
      "name": "height",
      "type": [
        "long",
        "null"
      ]
    },
    {
      "name": "weight",
      "type": [
        "long",
        "null"
      ]
    }
  ]
}

Now, imagine the two following objects.

obj = PyType(height=180, weight=None)
obj2 = PyType(height=180)

The schema would fit both the Avro representation, but does not let us distinguish the case in which weight is explicitly set to None from the case in which it is not set at all.

With this PR, we extend the types of the fields of a non-total TypedDict with string. This is going to be useful for all clients to mark a field as explicitly missing. Eveyrhing would make more sense if you look at the mentioned PR using this feature.

Addresses PNX-545