[chronik] Electrum: add the blockchain.headers.subscribe endpoint
Summary: Subscribe to get the new header when a block is connected.
Test Plan:
./test/functional/test_runner.py chronik_electrum_*
In terminal 1:
./src/bitcoind -regtest -debug=chronik -chronik -chronikscripthashindex -chronikelectrumbind="127.0.0.1:50001"
In terminal 2 (can be done in several terminals):
echo '{"jsonrpc": "2.0", "method": "blockchain.headers.subscribe", "params": [], "id": "test"}' | nc 127.0.0.1 50001 | jqin terminal 3:
./src/bitcoin-cli -regtest createwallet regtest ./src/bitcoin-cli -regtest -generate 1
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Subscribers: PiRK, tobias_ruck
Differential Revision: https://reviews.bitcoinabc.org/D18068