multiprocess: Add echoipc RPC method and test

Add simple interfaces::Echo IPC interface with one method that just takes and
returns a string, to test multiprocess framework and provide an example of how
it can be used to spawn and call between processes.

src/Makefile.am

@@ -159,6 +159,7 @@ BITCOIN_CORE_H = \
   init.h \
   init/common.h \
   interfaces/chain.h \
+  interfaces/echo.h \
   interfaces/handler.h \
   interfaces/init.h \
   interfaces/ipc.h \
@@ -563,6 +564,7 @@ libbitcoin_util_a_SOURCES = \
   compat/glibcxx_sanity.cpp \
   compat/strnlen.cpp \
   fs.cpp \
+  interfaces/echo.cpp \
   interfaces/handler.cpp \
   interfaces/init.cpp \
   logging.cpp \
@@ -815,6 +817,7 @@ if HARDEN
 endif
 
 libbitcoin_ipc_mpgen_input = \
+  ipc/capnp/echo.capnp \
   ipc/capnp/init.capnp
 EXTRA_DIST += $(libbitcoin_ipc_mpgen_input)
 %.capnp: