BP-69: Adopt slog for structured logging

[merlimat]

Master tracking issue for BP-69: Adopt slog for structured logging.

Summary

Adopt the slog library for structured logging across the BookKeeper codebase, replacing the ad-hoc SLF4J {}-style format strings. Retire the existing, barely-used bookkeeper-slogger module. Add an API for callers (Pulsar, applications, tools) to attach their own logging context attributes to a ledger Handle so cross-boundary log lines stay structured and correlated.

This mirrors the decision made upstream in Apache Pulsar PIP-467, whose migration is complete.

Proposal

See the full proposal at site3/website/src/pages/bps/BP-69-slog-structured-logging.md (PR forthcoming).

Implementation

Migration will be done in phases, one module per commit, each independently mergeable:

• Phase 1: bookkeeper-common
• Phase 2: stats, bookkeeper-common-allocator
• Phase 3: bookkeeper-server (core, largest phase)
• Phase 4: bookkeeper-http, tools
• Phase 5: stream/distributedlog
• Phase 6: metadata-drivers, benchmark, peripheral tests
• Phase 7: Cleanup — remove bookkeeper-slogger, remove @Slf4j usage
• Client API: withLoggerContext(Map<String, Object>) on CreateBuilder, OpenBuilder, DeleteBuilder

Sub-PRs will be linked here as they are opened.