Add an RPM backend for Linux

[freakboy3742]

What is the problem or limitation you are having?

While AppImage and Flatpak are formats that theoretically allow distribution-independent Linux "binaries" to be distributed, in practice they have proven to be problematic (e.g., #718, #919, #1029 and more).

An informal survey of preferences revealed that despite the need for root privileges, there is a significant portion of the user base that may prefer system packages for apps, rather than a user-space friendly packaging format.

Describe the solution you'd like

Briefcase should be able to generate an RPM package for an app specification.

Describe alternatives you've considered

Do nothing, and rely on AppImage/Flatpak.

Additional context

• .RPM packages should work across RedHat-based distributions (Fedora, RHEL, Centos, Alma) should all be able to share packages); They may also work with distributions with other bases (Suse?). This should be verified
• As we are primarily working with Python code, .RPM packages should work across distribution versions (e.g., a Fedora 30 package should work with Fedora 31), provided system packages are specified in ways that aren't version specific - e.g., specifying a libgtk-3 dependency should work on almost any version
• RedHat-based distributions all have Python3 interpreters in their default package repositories; however, that python version will change between distribution versions. Briefcase generally guarantees that the python version used to build an app is the version used to run the app; however, we may need to (optionally) relax that requirement to apps to specify a generic "Python3" dependency, rather than a specific "3.10" dependency.
• The manylinux specification should provide all the binary compatibility guarantees that are needed; as manylinux wheels have a very specific list of libraries they can link against, as long as the RPM package guarantees that subset, any installed wheel should (?) be binary compatible.

See also #1062, #1064