Add retry capability to REST client to handle transient errors

[Y-Sindo]

Add retry capability to REST client to handle transient errors. Tracking issue: Azure/azure-functions-signalrservice-extension#293

Add two retry modes:

1. Fixed delay
2. Exponential backoff delay （Do we need to add jitter to the delay?)

For message APIs:

Only the situations when HTTP status code > 500 are safe to retry.
For 500 error, runtime returns 500 when timeout, so it's not safe to retry.

For non-message APIs:

The following situations are considered to be transient:

1. HTTP response status code 5xx or 408(Request timeout).
2. Request timeout with the timeout length configured as ServiceManagerOptions.HttpClientTimeout

[vicancy]

Give the class and RetryMode a more specific naming, e.g. HttpClientRetryOptions?