Support HTTP response compression

[dkropachev]

Jira task: https://scylladb.atlassian.net/browse/DRIVER-626

Fixes #29

Summary

• add optional HTTP response compression; default Config advertises no Accept-Encoding and does not decode compressed responses
• replace the separate accepted-encoding knob and single decoder with decoder-owned encodings via Config::content_encoding_decoders
• add generic HttpContentEncodingDecoder API so each decoder advertises its accepted response encodings and decodes the specific matching content-encoding token
• validate decoder configuration: no null decoders, no empty advertised encodings, and each normalized encoding is owned by exactly one decoder
• add optional concrete ZlibContentEncodingDecoder for gzip/deflate when zlib is available; callers must configure it explicitly and can advertise a gzip/deflate subset
• decode discovery and DynamoDB AWS SDK HTTP-factory responses only when the response encoding is configured
• keep DynamoDB compression scoped to the HTTP factory path and only set Accept-Encoding for requests routed to Alternator nodes
• document optional zlib support, custom decoder usage, gzip+zstd composition, and the factory requirement for DynamoDB operations

API Surface

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

    [[nodiscard]] virtual std::vector<std::string> AcceptedResponseEncodings() const = 0;
    [[nodiscard]] virtual std::string Decode(std::string body, const std::string& content_encoding) const = 0;
};

class ZlibContentEncodingDecoder final : public HttpContentEncodingDecoder {
public:
    explicit ZlibContentEncodingDecoder(std::vector<std::string> accepted_response_encodings = {"gzip", "deflate"});

    [[nodiscard]] std::vector<std::string> AcceptedResponseEncodings() const override;
    [[nodiscard]] std::string Decode(std::string body, const std::string& content_encoding) const override;
};

struct Config {
    std::vector<std::shared_ptr<HttpContentEncodingDecoder>> content_encoding_decoders;
};

Examples:

scylladb::alternator::Config cfg;
cfg.content_encoding_decoders.push_back(
    std::make_shared<scylladb::alternator::ZlibContentEncodingDecoder>());

scylladb::alternator::Config cfg;
cfg.content_encoding_decoders.push_back(
    std::make_shared<scylladb::alternator::ZlibContentEncodingDecoder>(
        std::vector<std::string>{"gzip"}));
cfg.content_encoding_decoders.push_back(std::make_shared<ZstdDecoder>());

Testing

• make test-unit
• make check
• cmake -S . -B build-no-zlib-check -DCMAKE_DISABLE_FIND_PACKAGE_ZLIB=TRUE -DALTERNATOR_CLIENT_CPP_ENABLE_AWS=OFF -DCMAKE_CXX_FLAGS="-Wall -Wextra -Werror" && cmake --build build-no-zlib-check --parallel && ctest --test-dir build-no-zlib-check --output-on-failure -E Integration
• cmake -S . -B build-aws-check -DALTERNATOR_CLIENT_CPP_REQUIRE_AWS=ON fails locally because AWSSDK with the dynamodb component is not installed in this container