Add .NET 8 SDK chiseled container definition

[dviererbe]

Adds definition files and tests for .NET 8 SDK chiseled containers.

The chisel slice definitions need to land first:

• Add .NET 8 & 9 SDK chisel slice definitions for Ubuntu 25.04 canonical/chisel-releases#561
• Add .NET 8 & 9 SDK chisel slice definitions for Ubuntu 24.10 canonical/chisel-releases#562
• Add .NET 8 SDK chisel slice definitions for Ubuntu 24.04 canonical/chisel-releases#563

[rebornplusplus]

Hey @dviererbe, I am taking a look at the slice definitions now. Let's get those merged first and then we can take care of this one. :)

[cjdcordeiro]

Hi @dviererbe

thanks for this.

I believe this should be a rock.
The ones we have here in this repo are still Docker images because our release cadence has been following interim releases too, which is something we cannot yet do with Rockcraft. But once we do, we'll start building all subsequent .NET images as rocks too.