Modernize CI: Rust matrix, caching, and Solana BPF auto-test

[gruszkaa1]

Summary

This PR introduces a complete overhaul of the CI workflow to make development faster, safer, and easier to maintain across Rust and Solana versions.

What's new

• Multi-version Rust matrix (1.75.0 + stable)
• Automated Solana CLI setup (v1.16.1)
• Persistent cache for Cargo dependencies and build artifacts
• Native + BPF builds and tests run automatically
• Added cargo clippy linter for consistent code quality

Why

The template currently requires manual build and testing.
This workflow ensures:

• Continuous validation of Solana program compatibility
• Faster feedback loops during development
• Automatic checks before merging PRs

Validation

• ✅ Tested on Ubuntu 22.04 runners
• ✅ Builds successfully under both Rust 1.75.0 and stable
• ✅ BPF tests pass with Solana v1.16.1

Next Steps

• Add separate workflow for release builds (cargo build-bpf --release)
• Integrate audit step for dependency safety