Add block subscribe/unsubscribe websocket methods (#418) #855

Job	Run time
lint	1m 9s
tests (tests/integration/test_async_http_client.py)	1m 10s
tests (tests/integration/test_async_token_client.py)	1m 16s
tests (tests/integration/test_http_client.py)	1m 12s
tests (tests/integration/test_token_client.py)	1m 11s
tests (tests/integration/test_websockets.py)	2m 29s
tests (tests/integration/test_recent_performance_samples.py)	2m 2s
tests (tests/integration/test_memo.py)	1m 13s
tests (tests/unit)	28s
tests (src)	37s
test	1s
coverage	30s
	13m 18s

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