[WIP/running tests] POC: filestream growing fingerprint identity

[AndersonQ]

Add a proof-of-concept for a new "growing_fingerprint" file identity mode that addresses the limitation where files smaller than the fingerprint size (default 1024 bytes) cannot be tracked.

Key changes:

• Add growingFingerprintIdentifier that stores raw bytes (hex-encoded) instead of a hash, allowing the fingerprint to grow as the file grows
• Files can be tracked immediately regardless of size (no minimum threshold)
• Implement prefix matching: when a file grows, match the old (shorter) fingerprint as a prefix of the new (longer) fingerprint
• Add IterateOnPrefix() and UpdateKey() to the store for registry key migration
• Support in-place key updates without interrupting running harvesters
• Default max_length is 1000 bytes (matching OTEL's filelog receiver)

Configuration:

prospector.scanner:
  fingerprint.growing: true
  fingerprint.max_length: 1000  # optional, default 1000
  file_identity.growing_fingerprint: ~

This enables tracking small files that share initial content (e.g., common headers) by allowing their fingerprints to diverge as they grow with unique content.

Includes integration tests covering:

• Small files tracked immediately
• Files with identical initial content differentiated as they grow
• Fingerprint migration on file growth
• Restart scenarios

Open question/issue

Using the raw bytes (hex-encoded) as the fingerprint makes it easier to compare prefix matches, however increases the memory consumption, up to 1000 bytes per file per in-memory instance of the fingerprint. Also, it increases the storage used by the registry on disk.

Proposed commit message

Checklist

• [ ] My code follows the style guidelines of this project
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• [ ] I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• [ ] I have added an entry in ./changelog/fragments using the changelog tool.

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Author's Checklist

• [ ]

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git fetch upstream
git checkout -b 44780-growing-fingerprint upstream/44780-growing-fingerprint
git merge upstream/main
git push upstream 44780-growing-fingerprint