Add an infra issue template (#63730)

.github/ISSUE_TEMPLATE/infrastructure.md

+---
+name: I want to file a bug or feature request about Flutter's CI infrastructure
+about: As a Flutter contributor, you have run into problems with the build/test/release
+  infrastructure, including the build and performance dashboards
+  (http://flutter-dashboard.appspot.com), devicelab, LUCI (https://ci.chromium.org/p/flutter) etc.
+  Or you have new ideas around those systems.
+title: ''
+labels: 'team: infra'
+assignees: ''
+
+---
+
+<!--  Thank you for contributing to Flutter!
+
+      If you are filing a bug, please add the steps to reproduce, expected and actual results.
+
+      If you are filing a feature request, please describe the use case and a proposal.
+
+      If you are requesting a small infra task with P0 or P1 priority, please add it to the
+      "Infra Ticket Queue" project with "New" column, explain why the task is needed and what
+      actions need to perform (if you happen to know). No need to set an assignee; the infra oncall
+      will triage and process the infra ticket queue.
+-->