feat: define an adapter protocol version number constant

adapterlib/src/lib.rs

@@ -31,6 +31,11 @@ use crate::copy_tree::{CopyTreeError, copy_tree};
 
 mod copy_tree;
 
+/// Version of the broker/adapter protocol used.
+/// This gets put into every [`Request`] message so the adapter can
+/// detect its getting a message it knows how to handle.
+pub const PROTOCOL_VERSION: usize = 1;
+
 /// The type of a run identifier. For maximum generality, this is a
 /// string rather than an integer.
 ///