Enable dynamic templates to be pulled from the repository

[tanertopal]

Issue

Description

Pull template files from the repository.

In two follow-up PRs:

1. The templates directory will be copied to the root directory
2. The templates in the src/py/flwr/cli/new directory will be removed and the URL constant will be adjusted to the root directory.

Related issues/PRs

Proposal

Explanation

Checklist

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

Changelog entry

Any other comments?