Include all Briefcase tables in `pyproject.toml` during conversion

[rmartin16]

What is the problem or limitation you are having?

When an existing project is converted to use Briefcase, only the [tool.briefcase] and [tool.briefcase.app.<app name>] tables are populated. This makes it more difficult to take advantage of Briefcase's packaging controls because all of the those pyproject.toml tables must be created manually.

Describe the solution you'd like

Populate pyproject.toml with the platform-specific tables when converting an existing project. At the least, I see benefit from creating the empty tables; however, there are likely at least some keys that are worth exposing for all projects.

Describe alternatives you've considered

Manually create all platform tables in pyproject.toml.

Additional context

No response

[freakboy3742]

The thinking around omitting the tables was that we wouldn't be able to infer any default values, as we don't know the list of dependencies for an arbitrary app; but I agree that at least empty tables with placeholders for common requires, system_requires etc configurations would make sense.