Decode raw Rust V2 histograms

[Mirochill]

Summary:

• Detect a V2+Deflate external header when decoding raw bytes with b64_wrap=False and strip it before zlib decompression.
• Decode uncompressed V2 payloads without attempting zlib decompression, including the base64-wrapped form from the issue.
• Add regressions using the Rust V2DeflateSerializer and V2Serializer samples from error while decoding serialized histogram produced by rust version #29.

Addresses #29.

Validation:

• git diff --check
• git diff --cached --check
• git diff --check HEAD~1..HEAD
• Not run locally.