NuGet should add support for /runtimes/{rid}/nativeassets/{txm}/

[ericstj]

When we initially added the runtimes support this was excluded because corefx didn't have any scenarios for it. There are folks that need this. Native binaries can take a dependency on aspects of the {txm}, like WinRT surface area in the case of uap10.x, hosting-APIs/runtimes in the case of net4x.

We've discussed using/mis-using runtimes/{rid}/lib/{txm} for this but that won't work in all cases because the CLI treats native assets differently than lib and any native asset applicable for a core app must be in the native folder.

See https://github.com/dotnet/corefx/issues/8478.

[ericstj]

/cc @emgarten

[rrelyea]

@ericstj - after talking with @emgarten, we are leaning towards: "nativeassets".
We also discussed that /runtimes/{rid}/native/ would be treated as the "any" folder that we discussed on Friday.
Make sense?

[ericstj]

It's ok, the reason we said "assets" in the meeting was that folks in practice were using this folder for anything that they wanted to be RID specific but not have the lib extension filter applied. I'm ok with nativeassets.

[emgarten]

NuGet/NuGet.Client@aed1d51