feat(async): add asynchronous CoinGeckoAPI implementation

[arielen]

Changelog:

• Updated func_args_preprocessing decorator in utils.py to support async methods.
• Implemented AsyncCoinGeckoAPI class with asynchronous functionality mirroring CoinGeckoAPI.
• Added mock tests for all AsyncCoinGeckoAPI methods.
• Imported AsyncCoinGeckoAPI in __init__.py.
• Added dependencies: aiohttp, aiohttp_retry, and orjson in setup.py.
• Replaced json with orjson in api.py for better performance in CoinGeckoAPI.
• Updated README.md to include instructions for installing test dependencies.

BREAKING CHANGE:

• Tests now require pytest-asyncio and aioresponses.
• AsyncCoinGeckoAPI depends on orjson and aiohttp; orjson is significantly faster than the standard json module and should be considered for replacing it throughout CoinGeckoAPI.

[arielen]

Usage Example

import asyncio
from pycoingecko import AsyncCoinGeckoAPI

async def main():
    # Using the AsyncCoinGeckoAPI class via an async context manager
    # You can also initialize the key here
    async with AsyncCoinGeckoAPI() as api:
        price_data = await api.get_price(ids="bitcoin", vs_currencies="usd")
        print("Price data:", price_data)
        
        coins_category = await api.get_coins_categories()
        print("Coins categories:", coins_category)

    # Alternatively, using the create() method:
    obj_api = await AsyncCoinGeckoAPI.create()
    price_data = await obj_api.get_price(ids="bitcoin", vs_currencies="usd")
    print("Price data using create():", price_data)
    await obj_api.close()

if __name__ == "__main__":
    asyncio.run(main())

Explanation

• Async Context Manager:
  Using async with AsyncCoinGeckoAPI() as api: automatically initializes and later closes the session, ensuring proper cleanup without manual intervention.

• create() Method:
  You can also instantiate the API using the asynchronous factory method create(), which lets you manage the session manually (remember to call close() when done).

• Performance Benefits:
  The async implementation is significantly more efficient than the synchronous version. It allows you to perform multiple API calls concurrently and benefits from the speed of orjson for JSON parsing.

@man-c, always happy to interact and thanks for the review and further merge !