Add optional architecture input (x64, x86, arm64, amd64, arm, s390x, ppc64le, riscv64) and dedupe multi-version installs

[lmvysakh]

Description
This PR adds support for explicitly selecting the .NET SDK architecture in actions/setup-dotnet@v5 via a new optional architecture input, while preserving the installer’s native auto-detection behavior when the input is not provided.

Key changes

• New input: architecture added to action.yml (optional, supported values: x64, x86, arm64, amd64, arm, s390x, ppc64le, riscv64).
• Preserve auto-detect: When architecture is empty/unset, the action does not pass any --architecture / -Architecture flag to the install scripts, ensuring the install scripts continue to auto-detect the current runner architecture.
• Applies to all versions: When set, the specified architecture is applied consistently to each version from dotnet-version (including multiline input).
• Avoid duplicate installs: dotnet-version values are normalized (trimmed, empty lines removed) and treated as a set, preventing duplicate installations caused by repeated entries and/or whitespace differences.

Tests updated/added

• Validates that unsupported architecture values fails.
• Ensures duplicates in dotnet-version do not trigger extra installs.
• Verifies architecture flag is omitted when not set and included when set (x64/arm64).

Docs: README updated with usage examples and notes about auto-detection when architecture is not specified

- uses: actions/setup-dotnet@v5
  with:
    dotnet-version: |
      8.0.x
      9.0.x
    architecture: x64