[StaticWebAssets] Provide the ability to pass additional assets via an MSBuild property at build/publish time

[javiercn]

• The property will be a semicolon separated list of values where each value is a content root path, followed by a basepath and separated by comma.
  • For example `<>,<>;<>,<>.
  • We want to have a new target that runs at the same time as ResolveCurrentProjectStaticWebAssets and performs the same actions. Just with the set of provided content root base path pairs.
  • We need to add E2E tests for this at https://github.com/dotnet/sdk/tree/main/test/Microsoft.NET.Sdk.StaticWebAssets.Tests
  • Look at how is done in one of the static web assets baseline tests.
  • Use one of the assets from the existing testassets
    • add some folders outside of the project.
    • Run dotnet build passing the property (let's call it AdditionalContentRoots)
    • Validate that the assets are present in the generated manifests.
    • You can run the tests and inspect the manifest manually to make sure they work.