DRIVER-409 Add gzip request compression

[dkropachev]

Jira: https://scylladb.atlassian.net/browse/DRIVER-409
Closes #22

Summary

• Add a Config::request_compressor API and built-in GzipRequestCompressor, disabled by default unless configured.
• Make HttpRequestCompressor::Compress stream from request input to compressed output and return false when the compressor chooses to leave the request uncompressed.
• Keep the small-request policy inside GzipRequestCompressor: it skips bodies smaller than 1024 bytes by default, with GzipRequestCompressor(0) available to compress every body.
• Compress AWS SDK DynamoDB request bodies only for requests routed to known Alternator endpoints, setting Content-Encoding and exact Content-Length so compressed requests are not chunked.
• Preserve header optimization interop by keeping compression-required headers in the optimized allowlist, while skipped requests remain uncompressed with their original body and Content-Length.

Configuration API

scylladb::alternator::Config cfg;
cfg.request_compressor =
    std::make_shared<scylladb::alternator::GzipRequestCompressor>();

Custom compressors implement:

class HttpRequestCompressor {
public:
    virtual ~HttpRequestCompressor() = default;

    [[nodiscard]] virtual std::string ContentEncoding() const = 0;
    [[nodiscard]] virtual bool Compress(
        std::istream& input,
        std::uint64_t input_size,
        std::ostream& output) const = 0;
};

Returning false leaves the request uncompressed, which lets implementations make their own size or content-policy decision without adding more config fields.

Testing

• git diff --check
• cmake -S . -B build-check -DALTERNATOR_CLIENT_CPP_ENABLE_AWS=OFF
• cmake --build build-check -j$(nproc)
• ctest --test-dir build-check --output-on-failure
• cmake -S . -B build-no-zlib-check -DALTERNATOR_CLIENT_CPP_ENABLE_AWS=OFF -DCMAKE_DISABLE_FIND_PACKAGE_ZLIB=ON
• cmake --build build-no-zlib-check -j$(nproc)
• ctest --test-dir build-no-zlib-check --output-on-failure

AWS adapter unit and integration tests were added for request compression, small-body skip behavior, and header optimization interop, but could not be compiled locally because this environment does not have AWSSDK with the dynamodb component installed. Confirmed with cmake -S . -B build-aws-check -DALTERNATOR_CLIENT_CPP_REQUIRE_AWS=ON.