Extend schema to support incremental builds

[thompson-tomo]

NuGet Product(s) Involved

Other/NA

The Elevator Pitch

Improve support for incremental releases by introducing new attribute IncrementalBuild to simplify the lives of developers by enabling incremental builds to be hosted on nuget.org alongside other builds which will eliminate the need for additional nuget sources. This also helps with package source mapping.

Additional Context and Details

Nuspec should introduce a new attribute IncrementalBuild with a default value of false to signify that this version when used in conjunction with a preview version is an incremental build hence can be very unstable.

Incremental builds should be indicated on nuget.org in a similar manner to deprecated packages.

Nuget.org should limit the number of incremental builds hosted based on the major + minor version as well as age. Currently this process is overcome by relying on additional nuget feeds which doesn't play nicely with package source mapping.