Add basic record doc support

[BeRT2me]

This allows two different scenarios to work:

1. Pydantic Models with docstrings have their docstring converted to an Avro record doc.
2. Avro records with doc get added as Pydantic Models' docstring.

Aka:

class Test(AvroBase):
    """docstring"""
    c1: int

Becomes

{
    "type": "record",
    "namespace": "Test"
    "name": "Test",
    "doc": "docstring",
    "fields": [{"type": "int", "name": "c1"}]
}

And visa-versa.