Add Mergeable to contract utils

At this moment in time, `Mergeable` is defined in core
[here](https://github.com/dbt-labs/dbt-core/blob/1a5d6922dddf9ffe018d8860bb2f2141594ddbbe/core/dbt/contracts/util.py#L27).
We're curren't in the process of moving data artifacts of nodes defined
in dbt-core's `nodes.py` into dbt-artifacts. Some of these data artifacts
depend on `Mergeable`. We don't want artifacts to depend on core, thus
`Mergeable` has to be moved _somewhere_ upstream. Given `Mergeable`'s
similarity to `Replaceable` it made the most sense to move next to
`Replaceable` here in dbt-common.

dbt_common/contracts/util.py

@@ -5,3 +5,19 @@
 class Replaceable:
     def replace(self, **kwargs):
         return dataclasses.replace(self, **kwargs)
+
+
+class Mergeable(Replaceable):
+    def merged(self, *args):
+        """Perform a shallow merge, where the last non-None write wins. This is
+        intended to merge dataclasses that are a collection of optional values.
+        """
+        replacements = {}
+        cls = type(self)
+        for arg in args:
+            for field in dataclasses.fields(cls):
+                value = getattr(arg, field.name)
+                if value is not None:
+                    replacements[field.name] = value
+
+        return self.replace(**replacements)