conanfile: per-component build selection (c-api, node-exe, both)

[fpelliccioni]

Problem

Today a `conan create` on the kth recipe builds the full tree (domain + blockchain + consensus + database + network + node + node-exe + c-api + console tools). Downstream consumers have different needs:

• A Python / C# / WASM binding only needs `c-api` (the shared library + headers).
• A node operator only needs `node-exe` + its runtime deps, not the C-API layer.
• A library consumer that links against `kth_node` wants neither the exe nor the C-API, just the C++ libs.

Currently every downstream pays the full build cost (and gets the full install tree) even when they use a small slice.

Proposal

Add a Conan option (or a pair) to select which deliverables the build produces:

• `components=[c-api, node-exe, both]` (default `both` for backwards compatibility), OR
• two boolean options: `with_c_api` (default true) + `with_node_exe` (default true).

`conanfile.py::generate` / `build` / `package` forwards the selected components to CMake via `-DWITH_C_API=ON/OFF` + `-DWITH_NODE_EXE=ON/OFF` (or a single `-DKTH_COMPONENTS=...` string). The root `CMakeLists.txt` already has `WITH_CONSOLE` / `WITH_CONSOLE_CAPI` — extend the same pattern.

The default stays `both` so nothing existing breaks; consumers that want a faster / smaller build flip the options.

Why it matters

• py-native cold-builds today include the full node + consensus + network tree for a library that only links the c-api. Cuts significant CI minutes.
• Downstream CI matrices (cs-api, wasm, future bindings) all pay the same cost.
• Separates the "run a node" from "embed kth as a library" use cases more cleanly at the package-manager layer.

Notes

• Needs a pass through each `src//CMakeLists.txt` to guard `add_subdirectory` calls on the new toggles.
• The `c-api` sub-project already has `GLOBAL_BUILD` and `ENABLE_SHARED_CAPI` — those are orthogonal and stay as-is.
• Test plan: `conan create . --version=0.81.2 -o kth/:with_c_api=False` should produce a package with no `libkth-c-api.so` / c-api headers; same with `-o kth/:with_node_exe=False`.