github: Add config for Gemini code assist (#8873)

Add configuration to enable Gemini code reviews.

Source: https://github.com/grpc/grpc/blob/master/.gemini/config.yaml

RELEASE NOTES: N/A

Author: arjan-bal

.gemini/config.yaml

@@ -0,0 +1,13 @@
+have_fun: false
+memory_config:
+  disabled: false
+code_review:
+  disable: false
+  comment_severity_threshold: MEDIUM
+  max_review_comments: -1
+  pull_request_opened:
+    help: false
+    summary: false
+    code_review: false
+    include_drafts: false
+ignore_patterns: []