Add optional Retry Object to FuncXClient constructor

_As a funcX developer I may wish to customize how errors are retried so I can accommodate environmental problems in a manner appropriate to my application_

## Assumptions
1. Add a new argument to the funcX client constructor to accept a retry object
2. Make a reasonable default if user doesn't specify anything

Start by locating the FuncXClient constructor and reading how retry behavior is currently configured or applied. Define completion as accepting an optional retry object while preserving a reasonable default when none is supplied; the issue does not name specific files or tests to run.

Written by the indexing model from the issue text.