Add support for async credential providers (aiobotocore compatibility)

[mvribeiro75]

Problem

The current MSKAuthTokenProvider.generate_auth_token_from_credentials_provider() method only supports synchronous botocore credential providers.
When using aiobotocore with an async credential provider, calling:

credential_provider.load()

synchronously returns a coroutine object instead of actual credentials. This causes:

AttributeError: 'coroutine' object has no attribute 'access_key'

because the returned coroutine is never awaited.

---

How to Reproduce

Below is a minimal example showing the incompatibility.

import asyncio
from datetime import datetime, timedelta
from botocore.credentials import CredentialProvider
from aiobotocore.credentials import AioRefreshableCredentials
from aws_msk_iam_sasl_signer import MSKAuthTokenProvider

class AsyncMyCredentialProvider(CredentialProvider):
    CANONICAL_NAME = "async-app"

    async def load(self):
        await asyncio.sleep(0.1)  # Simulate async API call
        return AioRefreshableCredentials(
            access_key="AKIAEXAMPLE123",
            secret_key="wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY",
            token="example_session_token",
            expiry_time=datetime.now() + timedelta(hours=1),
            method="async-app",
            refresh_using=self._async_creds_fetcher,
        )

    async def _async_creds_fetcher(self):
        await asyncio.sleep(0.1)
        return {
            "access_key": "AKIAEXAMPLE123",
            "secret_key": "wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY",
            "token": "example_session_token",
        }

# Attempt to use async provider
async_provider = AsyncMyCredentialProvider()
region = "us-west-2"

token, expiry = MSKAuthTokenProvider.generate_auth_token_from_credentials_provider(
    region, async_provider
)

Result:

AttributeError: 'coroutine' object has no attribute 'access_key'

Why this happens:

1. generate_auth_token_from_credentials_provider() calls credential_provider.load() synchronously.
2. For async providers, .load() is a coroutine function, so calling it without await returns a coroutine object.
3. The MSK signer immediately tries to access .access_key on the coroutine object → boom 💥.

---

Current Workaround

You can wrap the async provider in a synchronous one:

from botocore.credentials import Credentials

class SyncWrapperProvider(CredentialProvider):
    def __init__(self, async_provider):
        self.async_provider = async_provider

    def load(self):
        creds = asyncio.run(self.async_provider.load())
        return Credentials(
            creds.access_key,
            creds.secret_key,
            creds.token
        )

Then pass SyncWrapperProvider to the MSK signer.

---

Proposed Solution

Add native support for async credential providers in MSKAuthTokenProvider (or provide an async variant, e.g., generate_auth_token_from_async_credentials_provider) so that applications using aiobotocore do not need custom sync wrappers.