Support function retries

There are many situations where a function may fail for various reasons. A simple way of overcoming intermittent failures is by retrying the function.

The main idea here is to
1. Extend the SDK so users can specify retries on errors
2. Extend the funcX service to retry failed executions up to some limit

The issue names two work areas: SDK retry configuration and funcX service retries for failed executions. Start by locating the SDK execution entry point and the service's failed-execution handling, then clarify retry limits, retryable errors, and completion criteria before changing either side.

Written by the indexing model from the issue text.