Add context local resource

[elina-israyelyan]

Closes Issue #728 , #760
Relates to #699

Added a ContextLocalResource class that inherits from Resource and overrides it so that the actual _resource and _shutdowner are stored and retrieved via ContextVar.
This allows each context (such as a FastAPI request) to have its own instance of the resource.
Since FastAPI creates a new context per request, each request now gets its own context-scoped resource.
Because the class extends Resource, the Closing marker continues to work as expected.

Demo
Run the following API, then do concurrent requests.

from dependency_injector import containers, providers
from dependency_injector.wiring import Closing, Provide, inject
from fastapi import Depends, FastAPI
from uuid import uuid4

global_list = []


class AsyncSessionLocal:
    def __init__(self):
        self.id = uuid4()

    async def __aenter__(self):
        print("entering session")
        return self

    async def __aexit__(self, exc_type, exc_val, exc_tb):
        print("closing session")

    async def execute(self, user_input):
        return "executing " + user_input


async def get_session() -> AsyncSessionLocal:
    print("start session")
    async with AsyncSessionLocal() as session:
        yield session
    print("close session")


app = FastAPI()


class Container(containers.DeclarativeContainer):
    db = providers.ContextLocalResource(get_session)


@app.get("/")
@inject
async def index(db: AsyncSessionLocal = Depends(Closing[Provide["db"]])):
    global global_list
    if db.id  in global_list:
        raise Exception("The db session is already being used")  # never reaches here
    global_list.append(db.id)
    res = await db.execute("SELECT 1")
    return str(res)


if __name__ == "__main__":
    import uvicorn

    container = Container()
    container.wire(modules=["__main__"])
    uvicorn.run(app, host="localhost", port=8000)
    container.unwire()

[ZipFile]

Hey! Thanks for contribution. Idea is good, but there are some code duplication issues needs to be addressed before we can merge it. Also, it would be really nice to have documentation updated with usage examples.