server_message: Create zod schema to parse message recieved from server.

The commit creates a `server_message.ts` module which
will home our `zod` schema for the messages received
from server. We can then import the schema to parse
the data in needed modules at the API boundaries.
This will also help us write better tests.

tools/linter_lib/custom_check.py

@@ -120,6 +120,7 @@ js_rules = RuleList(
                 "web/src/types.ts",
                 "web/src/util.ts",
                 "web/src/message_helper.ts",
+                "web/src/server_message.ts",
                 "web/tests/",
             },
             "exclude_pattern": "emails",