Configurable Retry Policy

Both `class blob_client` constructors initialize `executor_context blob_client::m_context` with the default `class retry_policy`. The default retry policy retries 26 times with an exponential back-off up to 60 seconds on each retry.

Please support a configurable retry policy. I'd suggest:
1. Allow `class retry_policy` to be constructed with both a maximum retry count and a retry interval.
2. Allow `class blob_client` to be constructed with an instance of `class retry_policy`.