feat: Modern RPC Server design

[ralflang]

Modern RPC Server Design

Remodels the old horde/rpc into a PSR-7 based Request Handler / Middleware approach.
Emits events for a PSR-14 EventListener instead of native logging/observability handling.
Currently implements soap (wsdl-less, backed by php-ext-soap) and json-rpc (now both for 1.1 and 2.0 protocols)
On top of JSON-RPC it also models a simple MCP protocol for tools and resources.

The new library is decoupled from Horde context, i.e. the Registry is now one of many pluggable API providers.
The Registry API provider may need to migrate into a separate package in a later stage but we are not there yet.

In middleware mode, the protocol handlers check if the incoming request looks like it is for them to handle and if not, they pass to the next middleware.
In handler mode, a request not matching protocol expectations emits an appropriate error response format.

Implementing ActiveSync and DAV Protocol Handlers in the same fashion needs some ground work in other areas first.

Documentation

See docs folder for architecture description