Can users set a fully custom User-Agent that completely replaces the default one, without appending the driver version?

### Use case
Security/compliance policies in some organizations require hiding internal client version information from server logs. Currently there is no way to do this from the client side.

### Describe the solution you'd like
Allow setting a fully custom User-Agent that completely replaces the default one, without appending the driver version.

### Describe the alternatives you've considered
When using the ClickHouse client to send requests to server, the User-Agent header always includes the driver version:User-Agent: ClickHouse-JavaClient/0.6.5 (OpenJDK 64-Bit Server VM/BiSheng_JDK_Enterprise_205.2.0.110.B001; Apache-HttpClient/5.3.4)

Using client_name can only prepend a custom prefix, but the driver version suffix is always appended at the end and cannot be removed.

### Additional context

Start by tracing how the Java client constructs the User-Agent header and how client_name is applied. Check the existing client request tests, then verify that a fully custom value replaces the default header without appending the driver version.

Written by the indexing model from the issue text.