Add PROXY protocol support

[blaubaer]

Reimplements gliderlabs/ssh#259 by @donovanhubbard for the current server lifecycle and configuration model.

Summary

• add optional PROXY protocol v1/v2 support using github.com/pires/go-proxyproto
• enable it through a single nullable Server.ProxyProtocol configuration field
• expose connection policy, header validation, read timeout, and buffer sizing
• apply processing consistently to both Serve and HandleConn before ConnCallback
• propagate proxied local and remote addresses to callbacks, contexts, and sessions
• add a standalone example and compile it in CI
• cover trust policies, validation failures, deadlines, shutdown, and configuration snapshots

Testing

• go test -count=1 ./...
• go test -race -count=1 ./...
• PROXY protocol tests with 20 normal repetitions
• go vet ./...
• golangci-lint run --color=never
• go test -mod=readonly ./... for all standalone example modules
• Linux, Windows, and Darwin amd64 cross-builds
• go mod tidy -diff and git diff --check