do not poll for availability if MSI type is not IMDS

[scott-the-programmer]

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Description

This PR tries to solve #684 by leveraging getMSIType() to ensure that we're not pinging for availability for non-IMDS endpoints (as this is should instead manifest itself as an auth error)

In an app service environment, the endpoint will be set to $MSI_ENDPOINT. If $MSI_ENDPOINT does not exist, it will assume that the msi type is IMDS and behave as it did previously

Validation in Azure

Validated in an Azure App Service by running

	authorizer, err := kvauth.NewAuthorizerFromEnvironment() 
	if err != nil {
		return nil, fmt.Errorf("key vault authorizer error: %w", err)
	}

[ghost]

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