Add automatic retry policy

[mattt]

This PR refactors the client's internal fetch method to automatically retry requests as appropriate (429 status code, as well as 5xx errors for GET requests). If the server sends a Retry-After header, that value is used. Otherwise, the function delays for a configured interval with exponential backoff and jitter.

This should all be in line with the retry logic in the Python client.

[mattt]

Blocked by test cases not supporting different responses.