Initial MLI schemas and MessageHandler class (#607)

This PR adds the Capnproto schemas and initial MessageHandler class and
tests.

Author: AlyssaCote

doc/changelog.md

@@ -9,6 +9,14 @@ Jump to:
 
 ## SmartSim
 
+### MLI branch
+
+Description
+
+- Added schemas and MessageHandler class for de/serialization of
+  inference requests and response messages
+
+
 ### Development branch
 
 To be released at some future point in time

setup.py

@@ -176,6 +176,7 @@ def has_ext_modules(_placeholder):
     "protobuf~=3.20",
     "jinja2>=3.1.2",
     "watchdog>=4.0.0",
+    "pycapnp==2.0.0",
     "pydantic==1.10.14",
     "pyzmq>=25.1.2",
     "pygithub>=2.3.0",