[RPM] Missing %post support and broken %postun implementation in MakeRPMConfig

[eximius313]

Description:
I found that the RPM packager implementation is missing critical functionality for executing scripts after installation and uninstallation.

Analyzing the source code in packages/flutter_app_packager/lib/src/makers/rpm/make_rpm_config.dart:

1. Missing %post support:
   The MakeRPMConfig class does not have a post field definition at all. This makes it impossible to define a post-installation script (needed for udev rules, permissions, etc.) via configuration.

2. Broken %postun support:
   The postun field exists in the MakeRPMConfig class, but it is ignored in the toJson() method. The %postun section in the Spec file generation is hardcoded, rendering the configuration field useless.

Source Code Reference:

• Missing Field: The class properties list prep, build, install, postun, but post is missing.
• Hardcoded Logic (toJson method):

// Current implementation ignores 'this.postun'
'%postun': ['update-mime-database %{_datadir}/mime &> /dev/null || :']
    .join('\n'),

Proposed Solution:

1. Add String? post; to the MakeRPMConfig class and its constructor/factory.
2. Map the configuration key (e.g., post_install_script) to this new post field in fromJson.
3. Update toJson to correctly inject both post and postun into the generated map, respecting the user's configuration.

// Expected behavior in toJson():
'%post': [
  'update-mime-database %{_datadir}/mime &> /dev/null || :',
  post ?? '',
].join('\n'),

'%postun': [
  'update-mime-database %{_datadir}/mime &> /dev/null || :',
  postun ?? '', // Use the class field instead of ignoring it
].join('\n'),