Add block subscribe/unsubscribe websocket methods #853

Job	Run time
lint	1m 10s
tests (tests/integration/test_async_http_client.py)	1m 10s
tests (tests/integration/test_async_token_client.py)	1m 17s
tests (tests/integration/test_http_client.py)	1m 9s
tests (tests/integration/test_token_client.py)	1m 16s
tests (tests/integration/test_websockets.py)	2m 29s
tests (tests/integration/test_recent_performance_samples.py)	2m 5s
tests (tests/integration/test_memo.py)	1m 25s
tests (tests/unit)	31s
tests (src)	35s
coverage	31s
test	1s
	13m 39s

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