feat: Added attribute to easily register additional lua methods

Previously this could only be done by implementing a trait, like
`AddAdditionalMethods`, that that has an add_methods function where you
can put your custom methods. With this new attribute you can pass in a
register function directly!

automation_macro/Cargo.toml

@@ -11,3 +11,6 @@ itertools = { workspace = true }
 proc-macro2 = { workspace = true }
 quote = { workspace = true }
 syn = { workspace = true }
+
+[dev-dependencies]
+mlua = { workspace = true }