[snyk-cli] Add standalone binary installation method

[brianpmccullough]

Proposal

Add an installMethod option to the existing snyk-cli feature:

• npm — current behavior and default for backward compatibility
• standalone — download Snyk's published standalone executable

Example:

{
  "features": {
    "ghcr.io/devcontainers-extra/features/snyk-cli:1": {
      "installMethod": "standalone",
      "version": "latest"
    }
  }
}

Motivation

The current feature always installs Snyk through npm. This makes installation
dependent on Node/npm and on npm registry configuration visible to the root user
during the feature build.

For example, an existing private-registry or authentication configuration can
cause the feature build to fail with npm E401, even though Snyk itself is a
public package.

Snyk officially publishes standalone executables for:

• Linux amd64
• Linux arm64
• Alpine amd64
• Alpine arm64

It also publishes corresponding .sha256 files.

Proposed behavior

• The standalone installation path would:
• Select the appropriate Linux/Alpine and amd64/arm64 artifact.
• Map version: latest to Snyk's stable release channel.
• Support pinned versions with or without a leading v.
• Download and verify the published SHA-256 checksum.
• Install the executable at /usr/local/bin/snyk.
• Avoid installing Node, npm, or Python.
• Preserve npm as the default installation method.
• Authentication would remain outside the feature installation.

Documentation

Standalone installation:
https://docs.snyk.io/developer-tools/snyk-cli/install-or-update-the-snyk-cli
Binary verification:
https://docs.snyk.io/developer-tools/snyk-cli/install-or-update-the-snyk-cli/verifying-cli-standalone-binaries