EPIC: Elevate our .NET SDK into a first class citizen

[viccuad]

Starting with .NET 8, a big chunk of the work from https://github.com/dotnet/dotnet-wasi-sdk made its way upstream. This means
it's a good time to revisit Kubewarden's .NET SDK for policies.

This is an EPIC to be converted into sub-issues.

Acceptance criteria

• Bump the SDK from .NET 7 to .NET 9.
• On the waPC library we use, WapcGuest, move from the now deprecated https://github.com/SteveSandersonMS/dotnet-wasi-sdk into the new wasi-experimental workload, which runs on a wasmtime host as the current .NET SDK, and other SDKs such as the Go or Rust ones.
• Implement all capabilities in the policy specification. Currently the SDK supports validating and mutating policies. Add Host capabilities following our host specification , including unit tests, tested with generated mocks. For this, other implementations such as the Go or Rust one can be used as reference. This includes:
  • Kubernetes context-aware capabilities
  • OCI capabilities
  • Network capabilities
  • Cryptographic capabilities
  • Signature verifier capabilities
• Document the SDK API using https://dotnet.github.io/docfx/index.html or similar.
• Create a policy template repository in https://github.com/kubewarden/dotnet-policy-template analogous to https://github.com/kubewarden/go-policy-template or https://github.com/kubewarden/rust-policy-template.
• Update documentation:
  • Refresh tutorial on writing C# policies: https://docs.kubewarden.io/tutorials/writing-policies/dotnet
  • Write blogpost to showcase the feature as usual (example).