ethexe-observer: subcribe blocks when creating observer

[techraed]

Problem to Solve

Observer subcsribes blocks only when events stream is created via async_stream::stream!. The problem, is that the async subscription rpc-task is done inside the macro, so first stream.await will actually spend time on subscribing blocks, intead of obtaining blocks and events.

Possible Solution

Subscribe in the new.

Notes

No response