Support extensions SEP-2133

SEP-2133 Extensions was recently approved through the SEP process.

This has resulted in a new 'extension' fields in ClientCapabilities and ServerCapabilities.

Note that the capabilities extension fields are not in the 11-25-2025 schema and are distinct from the 'experimental' field...which is in the 11/25/2025 and the current Java SDK.

Expected Behavior

Support the extension fields in ClientCapabilities and ServerCapabilities so that extensions can be made connect interoperable with similar extensions in other language sdks (e.g. python and typescript).

Current Behavior

Explained above.

Context

I am in the process of implementing an extension to support grouping of primitives I already have a similar extension in python and for typescript. To make python clients that support the extension to connect to java servers that support the groups extension it's necessary that the new capabilities extension field be supported by the Java SDK.

In the immediate term I'm working around this by using the existing 'experimental' capabilities field. But interoperability with other sdks won't work.

If a pr implementing this feature is desired, I would be willing and able to create one. Please LMK on this issue or in direct conversation if you would like me to contribute this.

Empieza por localizar las representaciones del Java SDK y las rutas de serialización de ClientCapabilities y ServerCapabilities. Compáralas con el esquema SEP-2133 enlazado en el issue y con las implementaciones de Python y TypeScript referenciadas. El trabajo estará terminado cuando ambos objetos de capacidades admitan campos de extensión interoperables sin confundirlos con campos experimentales.