feat: grpc 기능 deep 구현

src/grpc/grpc-core-api/build.gradle

@@ -0,0 +1,11 @@
+apply plugin: 'java-library'
+
+// The platform's port layer: identifiers, method policy, execution evidence, failure model,
+// deadline primitives and request context.
+//
+// No dependencies at all, and that is the contract rather than an accident. The Stable plan's
+// Global Constraints make `grpc-core-api` framework-free so "evidence and policy do not know about
+// a transport" is verifiable instead of aspirational — the same rule `messaging-core-api` holds.
+// A type here may not name io.grpc, Spring, Netty, protobuf or a database.
+dependencies {
+}