Add retry / backoff logic

[TomAugspurger]

Azure SDKs have built-in support for retrying and backoff based on the response status codes. This package should do the same.

Ideally, we would

1. Retry on 429 errors (too many requests) after the number of seconds in the message
2. Retry on 50x errors and other connection issues

https://github.com/Azure/azure-sdk-for-python/blob/main/sdk/core/azure-core/CLIENT_LIBRARY_DEVELOPER.md and https://devblogs.microsoft.com/azure-sdk/custom-transport-in-python-sdk-an-httpx-experiment/ have some background on the design of how Azure SDKs handle network I/O. I don't think we need a full-blown pluggable transport system yet, but we should gracefully handle retries.