Support idempotency on create export in the CLI.

[cleve-fauna]

Problem

The POST /v2/exports API supports idempotency, but this is not exposed in the CLI.

Solution

Expose it in the CLI for fauna export create via an optional --idempotency argument

Result

Customers can create idempotent exports if they desire.

Testing

Ran the tests also ran with the idempotency command locally against the production API.

You can see that:

• not using --idempotency is properly handled (idempotent_replayed: false is in the output)
• using it but not replaying is properly handled (idempotent_replayed: false is in the output)
• replaying is properly handled (idempotent_replayed: true is in the output)

This proves it works as a steal thread.

[jrodewig]

Left some nits on the help text. Feel free to disregard as desired.