Add Elsa ABP package descriptors and packaging #21761
BackDescription
Resolves https://github.com/abpframework/abp/issues/24973
Add Volo.Abp.Elsa.abpmdl to define package folders and add .abppkg descriptors for all Elsa projects (src and test). Update Volo.Abp.Elsa.Installer.csproj to include the .abpmdl and all .abppkg files as package content (Pack=true, PackagePath=content) so module metadata is included in the installer NuGet package.
Checklist
- I fully tested it as developer / designer and created unit / integration tests
- I documented it (or no need to document or I will create a separate documentation issue)
- I worked with the design team to get their idea for UI/UX design (or no need to design for this PR)
- I've assigned reviewer, related labels and set a milestone for this PR