transport/server: declare a new "CLIENT_OPTIONS" option as supported

Declare support for a 'CLIENT_OPTIONS' startup key.
This key is meant to be used by drivers for sending client-specific
configurations like request timeouts values, retry policy configuration, etc.
The value of this key can be any string in general (according to the CQL binary protocol),
however, it's expected to be some structured format, e.g. JSON.

Signed-off-by: Vlad Zolotarov <vladz@scylladb.com>

docs/dev/protocol-extensions.md

@@ -74,6 +74,8 @@ The keys and values are:
     as an indicator to which shard client wants to connect. The desired shard number
     is calculated as: `desired_shard_no = client_port % SCYLLA_NR_SHARDS`.
     Its value is a decimal representation of type `uint16_t`, by default `19142`.
+  - `CLIENT_OPTIONS` is a string containing a JSON object representation that
+    contains CQL Driver configuration, e.g. load balancing policy, retry policy, timeouts, etc.
 
 Currently, one `SCYLLA_SHARDING_ALGORITHM` is defined,
 `biased-token-round-robin`. To apply the algorithm,

transport/server.cc

@@ -1653,6 +1653,9 @@ std::unique_ptr<cql_server::response> cql_server::connection::make_supported(int
     opts.insert({"CQL_VERSION", cql3::query_processor::CQL_VERSION});
     opts.insert({"COMPRESSION", "lz4"});
     opts.insert({"COMPRESSION", "snappy"});
+    // CLIENT_OPTIONS value is a JSON string that can be used to pass client-specific configuration,
+    // e.g. CQL driver configuration.
+    opts.insert({"CLIENT_OPTIONS", ""});
     if (_server._config.allow_shard_aware_drivers) {
         opts.insert({"SCYLLA_SHARD", format("{:d}", this_shard_id())});
         opts.insert({"SCYLLA_NR_SHARDS", format("{:d}", smp::count)});