feat: optional TLS for P2P connections (#1420)

[iho]

Summary

Implements optional TLS for peer-to-peer connections (#1420).

• Encrypts P2P traffic with rustls (same stack as the HTTP API) for privacy against passive packet inspection
• Opt-in via p2p_config.tls_enabled (default false) so the existing plaintext network stays compatible
• Self-signed certificates are auto-generated under {data_dir}/p2p_tls/ when paths are not configured
• Clients intentionally do not verify peer certificates — goal is privacy encryption, not PKI authentication
• Optional PEM paths: tls_certificate_file / tls_certificate_key
• Config comments updated in grin-server.toml template

Peer discovery: Capabilities::TLS + tls_required

• New capability bit Capabilities::TLS (0b1_0000_0000), advertised when tls_enabled = true
• Not part of default capabilities (TLS remains opt-in for network compatibility)
• p2p_config.tls_required (default false): when set with tls_enabled, only attempt outbound connections to peers known to advertise TLS, and request PEER_LIST | TLS in peer-list queries
• Seeds, preferred, and allow-listed addresses are still tried for bootstrap (capabilities unknown until first successful handshake)
• Unknown peers (no stored capabilities) are skipped when tls_required is on

Design notes

• New Stream abstraction wraps plain TcpStream or a mutex-shared rustls session (needed because reader/writer threads cannot try_clone a TLS session the way they do TCP)
• Both peers must enable TLS to connect; mixed TLS/plaintext will fail the handshake
• Not consensus-breaking; pure transport layer change

How to enable

[server.p2p_config]
tls_enabled = true
# Optional: only connect to peers known to support TLS
# tls_required = true
# optional PEM paths:
# tls_certificate_file = "/path/to/p2p.crt"
# tls_certificate_key = "/path/to/p2p.key"

Testing

• cargo test -p grin_p2p (includes peer_handshake_tls, capability / tls_required helpers)
• cargo check -p grin_config -p grin_servers
• Manual: two nodes with tls_enabled = true connecting on testnet/local

Consensus / compatibility

• Not consensus breaking
• Default remains plaintext; operators must opt in
• Nodes with TLS enabled will not successfully complete Grin handshake with plaintext-only peers
• With tls_required = true, automatic outbound selection prefers the TLS peer mesh once capabilities are known

Closes #1420