feat: Add configurable bind address + Github Workflow improvements

[SeverinAlexB]

Bind address

• Add bind_address() method to DhtBuilder to configure which network interface DHT nodes bind to (defaults to 0.0.0.0)
• Add Testnet::builder() with a TestnetBuilder that defaults to 127.0.0.1 for better cross-platform compatibility (especially macOS)
• Update all tests to use localhost binding to avoid firewall prompts on macOS

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The key user-facing changes are:

1. New Dht::builder().bind_address(Ipv4Addr::LOCALHOST) API
2. New Testnet::builder(count).bind_address(...).build() pattern that defaults to localhost
3. The existing Testnet::new() methods still bind to 0.0.0.0 for backwards compatibility

This fixes #83

PR Github Workflow Improvements

• Split up existing jobs to run in parallel. This increases test speed substaintially
• Clippy check + some code fixes
• Minimal Rust Version check
• Run tests on Ubuntu, Windows, and Mac

[afterburn]

Do we need this? This looks like its just doing sync stuff

[afterburn]

@SeverinAlexB small nit but feel free to merge if you think its okay