Add pydantic field descriptions in prompts

[banana-bison]

Fixes: #5

In case of a pydantic description such as this:

class Address(BaseModel):
      street: str = Field(description="Street address")
      city: str = Field(description="City name")
      zip_code: str = Field(description="ZIP or postal code")

  class Person(BaseModel):
      name: str = Field(description="Full name of the person")
      age: int = Field(description="Age in years")
      email: str = Field(description="Email address")
      address: Address = Field(description="Home address")

The LLM would see a description like this:

1. `person` (Person): Person to process
      Fields:
      - name (str): Full name of the person
      - age (int): Age in years
      - email (str): Email address
      - address (Address): Home address
          - street (str): Street address
          - city (str): City name
          - zip_code (str): ZIP or postal code

Earlier it could see just person, not the nested fields that it contains.

[TomeHirata]

Let's remove _

[TomeHirata]

Let's move to top

[TomeHirata]

Which part do we expect to fail?

[banana-bison]

issubclass could raise an exception for inputs such as issubclass(Any, BaseModel). But since this is an internal function, I guess this is a little too defensive. I can remove it.

[TomeHirata]

Can we describe what the current prompt looks like and what it should be in the PR description?

[banana-bison]

@TomeHirata i've address all the comments and updated the PR description