Adopt PEP621 conventions for package metadata

[freakboy3742]

What is the problem or limitation you are having?

Briefcase uses a [tool.briefcase] section to specify project metadata. Many of the configuration items that Briefcase specifies have analogs in the PEP621 (currently maintained by the PyPA specification).

Describe the solution you'd like

Briefcase should honour PEP621 configuration keys where possible. This should be treated as another layer of cascading - so - a description specified in the PEP621 [project] section would be overridden by a value in the [tool.briefcase] section, which would be overridden by the value in the [tool.briefcase.app.myapp] section, and so on.

The briefcase new project template should generated PEP621 configuration items by default, rather than values in the [tool.briefcase] section.

This may require some adaptation for some keys - for example PEP621 allows authors to be a list, and LICENSE to be a dictionary. In these cases, Briefcase should be modified to accept PEP621-compatible keys.

Describe alternatives you've considered

Do nothing; Briefcase could continue to operate completely independently of PEP621.

Additional context

No response