blacklist the spanner API

For readers of the future, this appeared to be broken even before this
PR was introduced but I couldn't pinpoint exactly where the breakage was
introduced

Signed-off-by: Petros Angelatos <petrosagg@gmail.com>

etc/api/shared.yaml

@@ -11,6 +11,7 @@ api:
     # It seems functions are containing illegal characters, like `@context`
     - kgsearch
     - mapsengine
+    - spanner
     - firebaserules
     - servicemanagement
     # defines its own `Option` type