49832 Reland: Always use new plugin schema for plugin template (cla: yes, tool)

49834 [flutter_tools] Make Config context-free (cla: yes, tool, waiting for tree to go green)

49835 Roll engine 74d07b5f6b4f..e5209202fb00 (4 commits) (autoroller: commit, cla: yes)

49842 [flutter_tools] Migrate xcode_backend.sh to flutter assemble (cla: yes, tool, waiting for tree to go green)

49844 Be clearer about when and why we override HttpClient in tests (a: tests, cla: yes, framework, team)

49845 Add channel-specific dartpad URL to snippet generation (cla: yes, team, waiting for tree to go green)