Projection fails when reading list-of-struct data structures

[zpks]

The current implementation of pymongoarrow/schemas.py assings a projection of 'True' to all fields in a struct.
This does not work for structs that contain list-of-struct datatypes; this can be fixed by reworking the projection creation to use Mongo's dot notation.

Example:
read in a data field like
"a": {
"b": {
"c": [
{
"event": {
"$date": "2022-01-01T00:00:00.000Z"
},
"value": 2,
}
],
}
}

the projection to read this with a schema needs to be
{"a.b.c.event": True, "a.b.c.value": True}

The current implementation of the projection returns a projection of
{"a": {"b": {"c": {"event": True}}}, which fails as the Mongo client reads "b" as an operator.

[zpks]

Potential solution:

#285

[zpks]

Fixed by #285