Add generator support for `GenerateAdapters` annotation #256

Job	Run time
ensure-embedme	20s
check-score (hive)	1m 35s
ensure-codegen	2m 59s
code-quality (hive, stable)	1m 15s
check-score (hive_flutter)	2m 9s
code-quality (hive, beta)	1m 14s
check-score (hive_generator)	2m 34s
test-hive (vm, kernel)	58s
test-hive_flutter (stable)	55s
code-quality (hive_flutter, stable)	1m 29s
test-hive (chrome, dart2js)	3m 58s
test-hive_flutter (beta)	59s
code-quality (hive_flutter, beta)	1m 24s
test-hive (chrome, dart2wasm)	2m 51s
test-hive_generator (stable, ubuntu-latest)	3m 5s
code-quality (hive_generator, stable)	1m 17s
code-quality (hive_generator, beta)	1m 17s
test-hive_generator (stable, windows-latest)	2m 47s
test-hive_generator (beta, ubuntu-latest)	3m 26s
	36m 32s

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