Introduce circuit breaker manager

[Roman Glushko]

As brought up in #85, I would be great to have a single component that manages seamlessly all breakers in the service.

The manager could be defined somewhere as a singleton and passed around the places (via dependency injection, for example):

from typing import Any

import httpx


class CurrencyRate:
    ...


class CurrencyRates:
    def __init__(self, breaker_manager: CircuitBreakerManager) -> None:
        self._breaker_manager = breaker_manager

    async def get_rates(self) -> list[CurrencyRate]:
        with self.breaker_manager("currency.rates", exceptions=(httpx.NetworkError,)):  # manager creates/retrieves and activates breaker
            async with httpx.AsyncClient() as client:
                response = await client.get("https://api.github.com/events")

                return self._map_rates(response.json())

    def _map_rates(self, data: list[dict[str, Any]]) -> list[CurrencyRate]:
        ...