chore(java_templates): add default CODEOWNERS files for samples reviews (#588)

This should automatically request reviews from the googleapis/java-samples-reviewers team for changes to samples Java code.

If you provide a `codeowner_team` in the `.repo-metadata.json` configuration file, it also adds an entry for any Java code. Note that later entries in the file take precedence over earlier ones so samples Java code will only require reviews by the samples team.

Author: chingor13

synthtool/gcp/templates/java_library/.github/CODEOWNERS

@@ -0,0 +1,11 @@
+# Code owners file.
+# This file controls who is tagged for review for any given pull request.
+
+# For syntax help see:
+# https://help.github.com/en/github/creating-cloning-and-archiving-repositories/about-code-owners#codeowners-syntax
+{% if 'codeowner_team' in metadata['repo'] %}
+# The {{ metadata['repo']['codeowner_team'] }} is the default owner for changes in this repo
+**/*.java               {{ metadata['repo']['codeowner_team'] }}
+{% endif %}
+# The java-samples-reviewers team is the default owner for samples changes
+samples/**/*.java       @googleapis/java-samples-reviewers