Add CMake packages for schemas and C++ bindings

[fruffy]

Work in progress.

LLM (GPT Astra) text:

P4Runtime currently has no CMake package, so consumers such as P4C must locate its schemas themselves. This adds find_package(p4runtime CONFIG) with a relocatable schema directory and p4runtime::schemas, without requiring Protobuf or a compiler for schema-only use.

Optional C++ generation provides p4runtime::cpp for messages and p4runtime::grpc for service bindings. Installed consumers request COMPONENTS cpp or grpc; dependencies are discovered only for those components. Parent projects can reuse their existing Protobuf/gRPC targets, installation defaults off for subprojects, and cross builds can supply host code-generation tools.

Validation:

• Schema-only, C++, and gRPC builds, relocated installs, and add_subdirectory() consumers pass locally; the same matrix is added to CI.
• C++ and gRPC consumers compile, link, and run with distribution Protobuf 3.21.12 / gRPC 1.51.1.
• Schema-only and full gRPC integration checks also pass with CMake 3.16.9, exercising the minimum-version and top-level fallback paths.
• A CPM parent supplying Protobuf 25.3 builds and runs the C++ consumer.
• P4C's dependency discovery consumes the installed schema package without downloading P4Runtime.

Fixes #641.