Add Flower Client App connection error handling

[charlesbvll]

Issue

Description

When starting a long-running Flower client app, if anything interrupts the connection between the server and the client, the client will crash (making it not so long running...).

Related issues/PRs

N/A

Proposal

Explanation

Use the existing RetryInvoker class to wrap the communication function calls in order to catch any communication error and retry to call the given function after some time.

Checklist

• Implement proposed change
• Update the changelog entry below
• Make CI checks pass
• Ping maintainers on Slack (channel #contributions)

Changelog entry

The flower-client-app will now try to reconnect indefinitely to the flower-superlink in case of connection error. The arguments --max-retries and --max-wait-time can now be passed to the flower-client-app, --max-retries will define the number of tentatives the client should make before it gives up trying to reconnect to the Superlink, and, --max-wait-time defines the time before the client gives up trying to reconnect to the Superlink.

Any other comments?

N/A