Allow legacy field names in rosidl_generate_interfaces [rolling]

[Ryanf55]

Purpose

Allow camelCase in message fields.

Continuing discussion from https://github.com/ros2/rosidl/pull/834, with the PR now targetting rolling. As I have time, I'll satisfy the remaining actions.

Actions Remaining

• [ ] Add more descriptive author warnings
• [ ] Handle services and actions
• [ ] Test the effect of camelCase on tool X and Y
• [ ] Satisfy linters
• [ ] Add tests