Add a warning if logged in into multiple organizations

In https://github.com/Azure/azure-kusto-python/blob/337e4b77116b9316e617a33415068327dc0e6064/azure-kusto-data/azure/kusto/data/_token_providers.py:

folder = os.getenv("AZURE_CONFIG_DIR", None) or os.path.expanduser(os.path.join("~", ".azure"))
token_path = os.path.join(folder, "accessTokens.json")
with open(token_path) as f:
    data = json.load(f)

# TODO: not sure I should take the first
return data[0]

In case accessTokens.json contains multiple tokens, it happens that you grab the wrong credential. Raise a warning in case data has multiple elements, s.t. the user can simplify the reason for failed query.

[yogilad]

@kopytjuk, we had an internal discussion regarding the current implementation. Bottom line is, it was devised when there was no official integration for az-cli, and it works by delving deep into the private implementation of az. Today Azure.Identity allows for this authentication mode, therefore we will scrap our private implementation in favor of Azure.Identity.

That being said, Azure.Identity will also return one of the logged in accounts token. You may control the account being used with the az account set command. More information on az account can be found here: https://docs.microsoft.com/en-us/cli/azure/account?view=azure-cli-latest

[yogilad]

PR #318