Merge bitcoin/bitcoin#22218: multiprocess: Add ipc::Context and ipc::capnp::Context structs

3e33d170cc Add ipc::Context and ipc::capnp::Context structs (Russell Yanofsky)

Pull request description:

  These are currently empty structs but they will be used to pass some function and object pointers from bitcoin application code to IPC hooks that run, for example, when a remote object is created or destroyed, or a new process is created.

  ---

  This PR is part of the [process separation project](https://github.com/bitcoin/bitcoin/projects/10). The commit was first part of larger PR #10102.

ACKs for top commit:
  ariard:
    Code Review ACK 3e33d170

Tree-SHA512: fd949fae5f1a973d39cb97f2745821ab2f62b98e166e53bc2801f97dcde988e18faaaaa0ffc2a82c170938b3a18078b6162fa35460e6e7c635e681b3c9e5b0a6

src/interfaces/ipc.h

@@ -9,6 +9,10 @@
 #include <memory>
 #include <typeindex>
 
+namespace ipc {
+struct Context;
+} // namespace ipc
+
 namespace interfaces {
 class Init;
 
@@ -58,6 +62,9 @@ public:
         addCleanup(typeid(Interface), &iface, std::move(cleanup));
     }
 
+    //! IPC context struct accessor (see struct definition for more description).
+    virtual ipc::Context& context() = 0;
+
 protected:
     //! Internal implementation of public addCleanup method (above) as a
     //! type-erased virtual function, since template functions can't be virtual.