Attribute based manifest system

[dazinator]

as discussed in #58

We should add a new feature to DnnPackager to allow an attribute based Manifest system.

Following is current suggestion of the fundamentals for how this can work:

1. Install DnnPackager to your project as usual. Everything remains the same as before.
2. To start using the attribute system, In the Package Manager Console type Install-Attributes
   • This will add the source code files for the packaging attributes to your project so that they get compiled within your own assembly which eliminates the need to distribute a third party assembly / dependency with your module.
3. You can now prefix your classes with the appropriate attributes (yet to be defined)
4. During build, DnnPackager will reflect through the classes in the output assembly, discovering the attributes (by attribute typename) - and it will then suppliment the existing manifest file with that information.

The information from attributes will suppliment the existing manifest file, not replace it completely.

For example, if you have an existing manifest with a package xml element in, and then you use the attributes system to declare another package - this result in a manifest.xml file with both packages in.

UPDATE: Another possiblity, rather than adding the attribute source code to the project, is to use a Fody Weaver, like JetBrains does for resharper: https://github.com/Fody/JetBrainsAnnotations