feat(template): Add template for singleton protocols.

Separate templates and add one for protocols with only a single contract.

substreams/ethereum-template-singleton/buf.gen.yaml

@@ -0,0 +1,12 @@
+
+version: v1
+plugins:
+- plugin: buf.build/community/neoeinstein-prost:v0.2.2
+  out: src/pb
+  opt:
+    - file_descriptor_set=false
+
+- plugin: buf.build/community/neoeinstein-prost-crate:v0.3.1
+  out: src/pb
+  opt:
+    - no_features