Add generator support for `GenerateAdapters` annotation #234

Job	Run time
ensure-embedme	9s
check-score (hive)	1m 36s
ensure-codegen	3m 18s
code-quality (hive, stable)	1m 11s
check-score (hive_flutter)	2m 2s
code-quality (hive, beta)	1m 15s
check-score (hive_generator)	2m 8s
test-hive (vm, kernel)	56s
code-quality (hive_flutter, stable)	1m 27s
test-hive_flutter (stable)	53s
test-hive (chrome, dart2js)	4m 23s
code-quality (hive_flutter, beta)	1m 20s
test-hive_flutter (beta)	51s
test-hive (chrome, dart2wasm)	2m 42s
test-hive_generator (stable, ubuntu-latest)	4m 13s
code-quality (hive_generator, stable)	1m 20s
test-hive_generator (stable, windows-latest)	5m 15s
code-quality (hive_generator, beta)	1m 15s
test-hive_generator (beta, ubuntu-latest)	4m 3s
	40m 17s

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