closes https://github.com/paritytech/parity-bridges-common/issues/2982
closes https://github.com/paritytech/parity-bridges-common/issues/2730

The main change is in the
bridges/relays/lib-substrate-relay/src/finality/target.rs, changes in
other files are just moving the code

~I haven't been able to run zn tests locally - don't know why, but it
keeps failing for me locally with: `
Error running script:
/home/svyatonik/dev/polkadot-sdk/bridges/testing/framework/js-helpers/wait-hrmp-channel-opened.js
Error: Timeout(300), "custom-js
/home/svyatonik/dev/polkadot-sdk/bridges/testing/framework/js-helpers/wait-hrmp-channel-opened.js
within 300 secs" didn't complete on time.`~ The issue was an obsolete
`polkadot-js-api` binary - did `yarn global upgrade` and it is ok now