feat(framework:skip) Allow installing flwr via a direct reference

[Robert-Steiner]

Issue

Description

• Adds the ability to install the flwr package via a direct reference.

Examples:

# main branch
--build-arg FLWR_VERSION_REF=git+https://github.com/adap/flower.git@main

# commit hash
--build-arg FLWR_VERSION_REF=git+https://github.com/adap/flower.git@1187c707f1894924bfa693d99611cf6f93431835

# tag
--build-arg FLWR_VERSION_REF=git+https://github.com/adap/flower.git@v1.11.0

# artifact store
--build-arg FLWR_VERSION_REF=https://artifact.flower.ai/py/main/latest/flwr-1.11.0-py3-none-any.whl

Related issues/PRs

Proposal

Explanation

Checklist

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

Any other comments?