[Chronik] Add Chronik, connect it to CValidationInterface

Summary:

1. Add Chronik, which will contain all the data/handles required to run Chronik.
2. Make setup_bridge return a boxed Chronik.
3. Add ChronikValidationInterface, to capture all the tx/block events happening in bitcoind.
4. Add Start-/StopChronikValidationInterface to create a Chronik and start + register / stop + unregister ChronikValidationInterface.

Test Plan:

1. ninja
2. ./src/bitcoind -regtest -reindex -debug=chronik -> displays "Chronik: block connected" (for the genesis block)
3. In a new window ./src/bitcoin-cli -regtest generatetoaddress ecregtest:qqmuvc70ttys4lg8se5tt7rgk53q6cmthuamcuaexg -> displays

"Chronik: block connected" in bitcoind

4. ./src/bitcoin-cli invalidateblock <hash> -> displays "Chronik: block disconnected" in bitcoind
5. Can also test adding/removing tx to/from mempool

A proper python-based HTTP test will be added in future diffs.

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12035