Add generator support for `GenerateAdapters` annotation #175

Job	Run time
code-quality (stable)	1m 9s
check-score (hive)	1m 39s
ensure-codegen	3m 10s
code-quality (beta)	54s
check-score (hive_generator)	2m 16s
ensure-embedme	6s
check-score (hive_flutter)	2m 3s
test-hive_flutter (stable)	50s
test-hive (vm, kernel)	53s
test-hive_flutter (beta)	52s
test-hive (chrome, dart2js)	4m 17s
test-hive_generator (stable, ubuntu-latest)	4m 9s
test-hive (chrome, dart2wasm)	2m 48s
test-hive_generator (stable, windows-latest)	5m 28s
test-hive_generator (beta, ubuntu-latest)	4m 3s
	34m 37s

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