[client-v2] Build the User-Agent in the standard client format and let applications add metadata #3202

client-v2 always appends its own part to the application's client name (HttpAPIClientHelper#buildDefaultUserAgent / #correctUserAgentHeader, same in 0.9.5 and 0.11.0-rc1):

<client_name> clickhouse-java-v2/<v> (<os.name>; jvm:<java.version>) Apache-HttpClient/<v>

The standard ClickHouse client format, already used by clickhouse-go, clickhouse-js, clickhouse-connect and clickhouse-rs, is:

<app_name>/<app_version> <client_name>/<client_version> (<comment>; <key1>:<value1>; <key2>:<value2>)

Needed

1. Send the language version as lv:java/<java.version> instead of jvm:, and the OS as os:<os.name> instead of a plain comment.
2. Don't append Apache-HttpClient/<v> after the bracket group (drop it or move it into a key).
3. Let applications add key/value pairs to the client's bracket group, e.g. Client.Builder#setClientMetadata(Map<String, String>). Today a connector has to add its own bracket group before clickhouse-java-v2/<v>, so it can't follow the format.

Example (Flink connector)

• Now: Flink-ClickHouse-Sink/0.2.0 (fv:flink/2.0.0; api:table) clickhouse-java-v2/0.9.5 (Mac OS X; jvm:17.0.19) Apache-HttpClient/5.4.4
• Expected: Flink-ClickHouse-Sink/0.2.0 clickhouse-java-v2/0.11.0 (fv:flink/2.0.0; api:table; lv:java/17.0.19; os:Mac OS X)

This affects every Java-based connector that sets a client name (Flink, Spark, Kafka Connect).