Generic queries

[anand2312]

Adds support for:

class Country(BaseModel):
    __table_name__: typing.ClassVar[str] = "countries"
    # fields follow

r = client.from_model(Country).select("*").execute()
reveal_type(r.data)  # Type is List[Country]

More examples

from typing import ClassVar, List, Optional
from datetime import datetime

from pydantic import BaseModel
from postgrest_py import SyncPostgrestClient

class City(BaseModel):
    id: Optional[int]
    created_at: Optional[datetime]
    country_id: Optional[int]
    city_name: Optional[str]

    # if this classvar is set on a model, that is used in queries instead of the model name
    __table_name__: ClassVar[str] = "cities"

class Country(BaseModel):
    id: Optional[int]
    created_at: Optional[datetime]
    country_name: Optional[str]
    capital: Optional[str]
    cities: Optional[List[City]]  # allows to use this model for joins between the tables

    __table_name__: ClassVar[str] = "countries"

client = SyncPostgrestClient(...)

r = client.from_("countries").select("*,cities(*)").execute()
reveal_type(r.data)  # Type is List[Dict[str, Any]], as no model was passed

# this will work for joins as well, as we have included the cities field in the Country model
r = client.from_("countries").select("*,cities(*)").execute(model=Country)
reveal_type(r.data)  # Type is List[Country]

r = client.from_model(Country).select("*,cities(*)").execute()
reveal_type(r.data)  # Type is List[Country]

Note that another model can not be passed to .execute if client.from_model is used (this is to make it play well with the type system; if anyone has any alternate implementation ideas please let me know!)
That is,

client.from_model(Country).select("*").execute(model=FooBar)  # error!

Pros

Works with the type system (tested with pyright)
Gets good editor autocompletion (tested on vscode)

Cons

Request builder classes had to be duplicated to make generic versions that would allow this.

Please let me know if you have other ideas for implementing this feature!