feat(compression): Added pure rust compression codecs to avoid C-backed crates

[LorenzoTettamanti]

Related issue: #420

Changes

Replaces the C-backed compression crates with pure-Rust implementations, so
that consumers of pulsar-rs no longer link liblz4 / libzstd at all:

Codec	Before	After
lz4 (lz4)	lz4-sys (C)	lz4_flex 0.11 (pure Rust)
zstd (zstd)	zstd-sys (C)	ruzstd 0.9 (pure Rust)
zlib (flate2)	already pure Rust	unchanged
snappy (snap)	already pure Rust	unchanged

• CompressionLz4::mode removed: lz4_flex doesn't expose a compression
  mode; the mode field had no effect anymore. LZ4 compression now always
  uses the default lz4_flex::block::compress behavior.
• CompressionZstd::level changed from i32 to
  ruzstd::encoding::CompressionLevel. Only Fastest and Uncompressed
  are currently implemented upstream (Default/Better/Best panic in
  ruzstd 0.9);