verifier_tools: Support prefetching and local caching of log entries.

[billy-lau]

cmd/verifier/verifier.go:

• added --fetch_entries flag to pre-fetch and cache entries locally up to the current checkpoint without requiring a payload or running inclusion proofs.
• added --concurrency flag to configure worker threads for Tessera tiles.
• added --cache_dir flag to specify a custom root directory for the cache.
• updated error handling and flag.Usage to improve usability.

internal/tiles/reader.go:

• added log entry prefetching utilities for both legacy and Tessera logs, supporting bounded concurrency and incremental caching of immutable tiles.
• introduced configurable cache directory management with automatic eviction of stale partial tiles.
• modernized HTTP transport with connection pooling, timeouts, and context-aware cancellation.

internal/tiles/reader_test.go:

• added TestFetchAllLegacyEntries testing multi-file caching and cache hits.
• added TestFetchAllTesseraEntries testing concurrent tile downloads, skip logic, and error/cancellation handling.
• added TestSetCacheDir verifying custom cache directory redirection.
• hermetically isolated test caches across platforms ($HOME and $XDG_CACHE_HOME).

README.md:

• documented Verification Mode and Pre-fetching & Offline Cache Mode.
• added flags reference table covering new flags and aliases.

Change-Id: I411b1587280aaf0df03915ab946baf7b5076cf4a