feat(framework) Add initial SuperExec service

[charlesbvll]

Issue

Description

The SuperExec service will be responsible of receiving StartRun requests from the flwr run CLI, passing this request to a given Executor plugin, and returning the response to the flwr run CLI.

Related issues/PRs

N/A

Proposal

Explanation

• Add initial version of this service under src/py/flwr/superexec with a dummy test Executor in test_executor.py.
• Modify the flwr run command to accept a --use-superexec argument to be able to test the service.

Checklist

• Implement proposed change
• Write tests
• Make CI checks pass
• Ping maintainers on Slack (channel #contributions)

Any other comments?

N/A