Add a configurable RetryPolicy to the HereAccessTokenProvider, that by default retries socket and 5xx errors with backoff

[kenmccracken]

Add a configurable RetryPolicy to the HereAccessTokenProvider, that by default retries socket and 5xx errors up to 2x with backoff.

This would allow, particularly when using HereAccessTokenProvider.Builder.setAlwaysRequestNewToken(true), some additional client-side resilience to network hiccups or expected low-probability failures server-side.

The DefaultRetryPolicy can then be chosen from java.util.Random using exponential random backoff starting with [0,200] ms, with up to 3 max attempts. Or, other suitable alternative.