Add explicit architecture and multi-architecture support for .NET SDK installation

[lmvysakh]

This PR implements support for specifying the target architecture when installing the .NET SDK using the setup-dotnet Action.

• Adds an architecture input, allowing users to select x64, arm64 for all installs (default: x64).
• Extends the dotnet-version input to optionally specify per-line architecture, supporting multi-arch installs in one step.
• Updates parsing, install logic, and documentation to clearly describe usage and new (version: 8.0.x, arch: arm64) formats.
• Enhances the README and action.yml to document usage and supported syntax.