Read OAuth2 client secret from configured file

[michaeljmarshall]

The main context for this feature is here: https://github.com/datastax/pulsar-heartbeat/pull/23#discussion_r815143257.

Essentially, because we configure the Pulsar Heartbeat via a config map in kubernetes, it'd be good to provide the client secret via a separate file so that it can be stored as a secret in k8s. We'll need an additional configuration in the tokenOAuthConfig to make the client secret's file configurable.