perf: reuse zstd compressors in encoding

[wkalt]

Prior to this commit, the ZstdBufferCompressor would construct a new
zstd stream encoder on every call to compress. With this change we
create one compression context for the ZstdBufferCompressor, and reuse
it across calls to compress.

Reuse is not implemented for lz4 compression or for decompression. These
were both explored but did not bring meaningful benefits over the
existing code.

[wkalt]

Here is the improvement from the encoding benchmark the patch introduces:

[/mnt/work/home/wyatt/work/lance] $ cargo bench --bench encoder -- encode_compressed --baseline encode-before
   Compiling lance-encoding v2.0.0-beta.5 (/mnt/work/home/wyatt/work/lance/rust/lance-encoding)
    Finished `bench` profile [optimized + debuginfo] target(s) in 19.80s
     Running benches/encoder.rs (target/release/deps/encoder-a8754c30e2dc3932)
Benchmarking encode_compressed/zstd_strings_10cols: Warming up for 3.0000 s
Warning: Unable to complete 10 samples in 5.0s. You may wish to increase target time to 58.3s.
encode_compressed/zstd_strings_10cols
                        time:   [5.6452 s 5.6886 s 5.7446 s]
                        thrpt:  [8.7038 Melem/s 8.7895 Melem/s 8.8571 Melem/s]
                 change:
                        time:   [-30.007% -29.406% -28.676%] (p = 0.00 < 0.10)
                        thrpt:  [+40.205% +41.655% +42.872%]
                        Performance has improved.
Found 2 outliers among 10 measurements (20.00%)
  1 (10.00%) high mild
  1 (10.00%) high severe

[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ust/lance-encoding/src/encodings/physical/block.rs	77.77%	6 Missing and 2 partials ⚠️

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