chore: add default assignee for issues (#1065)

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:
- [ ] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/java-spanner/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [ ] Ensure the tests and linter pass
- [ ] Code coverage does not decrease (if any source code was changed)
- [ ] Appropriate docs were updated (if necessary)

Author: zoercai

.github/blunderbuss.yml

@@ -1,5 +1,7 @@
 # Configuration for the Blunderbuss GitHub app. For more info see
 # https://github.com/googleapis/repo-automation-bots/tree/master/packages/blunderbuss
+assign_issues:
+  - thiagotnunes
 assign_prs_by:
 - labels:
   - samples

synth.py

@@ -91,4 +91,5 @@
     '.github/CODEOWNERS',
     '.github/sync-repo-settings.yaml',
     '.github/release-please.yml',
+    '.github/blunderbuss.yml',
 ])