util: expose an "available" API on compression engines

When the zstd compression engine is introduced, it won't work in all
installations, namely pure Python installs. So, we need a mechanism to
declare whether a compression engine is available. We don't want to
conditionally register the compression engine because it is sometimes
useful to know when a compression engine name or encountered data is
valid but just not available versus unknown.

mercurial/util.py

@@ -3047,6 +3047,15 b' class compressionengine(object):'
         """
         raise NotImplementedError()
 
+    def available(self):
+        """Whether the compression engine is available.
+
+        The intent of this method is to allow optional compression engines
+        that may not be available in all installations (such as engines relying
+        on C extensions that may not be present).
+        """
+        return True
+
     def bundletype(self):
         """Describes bundle identifiers for this engine.