Publish expected native program entrypoint in sdk/

sdk/src/native_program.rs

@@ -0,0 +1,8 @@
+use account::KeyedAccount;
+
+// All native programs export a symbol named process()
+pub const ENTRYPOINT: &str = "process";
+
+// Native program ENTRYPOINT prototype
+pub type Entrypoint =
+    unsafe extern "C" fn(keyed_accounts: &mut [KeyedAccount], data: &[u8]) -> bool;