fix: made .clang-format generic for any repo (#1454)

As we prepare to move to a monorepo, we want all of our existing repos
to be formatted with the same .clang-format config. This config removes
any per-library specific names (i.e., I removed "spanner"). Instead,
this names the headers and directories of the *common* headers and
ensures that those sort after the other quoted "google/cloud/.+"
includes, which should be the project ones.

Author: devjgm

.clang-format

@@ -9,10 +9,11 @@ PointerAlignment: Left
 
 IncludeBlocks: Merge
 IncludeCategories:
-- Regex: '^\"google/cloud/spanner'
-  Priority: 500
-- Regex: '^\"google/cloud/'
+# Matches common headers first, but sorts them after project includes
+- Regex: '^\"google/cloud/(internal/|grpc_utils/|testing_util/|[^/]+\.h)'
   Priority: 1500
+- Regex: '^\"google/cloud/'  # project includes should sort first
+  Priority: 500
 - Regex: '^\"'
   Priority: 1000
 - Regex: '^<grpc/'