Make `CredentialHandler` classes compatible with managed identity command line login (`az login --identity`)

[dylanhmorris]

Currently it assumes base az login (which defaults to browser-based interactive authentication)

[dylanhmorris]

@SamuelBrand1 has a reprex here. I believe @jkislin may have also encountered this.

[SamuelBrand1]

Here is a reprex:

• Login using managed identity

$ az login --identity

• Set up suitable env variables @dylanhmorris has a bash script for this and I use .env
• Try to construct a client

import dotenv

dotenv.load_dotenv()

from azuretools.auth import EnvCredentialHandler, get_sp_secret

creds =EnvCredentialHandler() #credentials from env vars
get_sp_secret(vault_url=creds.azure_keyvault_endpoint, vault_sp_secret_id=creds.azure_keyvault_sp_secret_id)

fails with error

ChainedTokenCredential failed to retrieve a token from the included credentials.
Attempted credentials:
AzureCliCredential: ERROR: Tenant shouldn't be specified for managed identity account