Add checksum validation for Wasm SDK downloads

[leogdion]

Context

Currently downloading Wasm SDKs from Swift.org without checksum validation (line 808-824 in action.yml).

Blocker

Swift.org does not currently publish checksums for Wasm SDK artifact bundles.

Security Benefit

Prevents supply chain attacks via tampered SDK downloads.

Implementation

Add checksum verification before swift sdk install when checksums become available.

Reference

Swift.org download pattern and SDK installation at action.yml:806-824

Current Code Location

# action.yml:808
# TODO: Add checksum validation when Swift.org publishes checksums for Wasm SDKs
SDK_URL="https://download.swift.org/swift-${SWIFT_VERSION}-release/wasm-sdk/swift-${SWIFT_VERSION}-RELEASE/${SDK_NAME}.artifactbundle.tar.gz"

Related

Part of PR review recommendations for #81 (1.5.0 release)