feat: prewitnessing uses unfinalised sc stream

[msgmaxim]

Pull Request

Closes: PRO-929

Checklist

Please conduct a thorough self-review before opening the PR.

• I am confident that the code works.
• I have updated documentation where appropriate.

Summary

Pretty straightforward change. Tested on localnet and seems to behave as expected. The only thing that's still unclear to me is whether we also want to specialise is_header_ready somehow so that we don't pessimistically wait for the next block (which could be a long time for BTC). Figured it is worth discussing this before making more changes.

[linear]

PRO-929 Use unfinalised stream for prewitnessing

See: #4146

We want to remove the delay that we have as described in PRO-923 by using the unfinalised stream for the pre-witnessers.

[codecov]

Codecov Report

Merging #4220 (33b58e3) into main (22c713b) will decrease coverage by 0%.
The diff coverage is 0%.

@@          Coverage Diff          @@
##            main   #4220   +/-   ##
=====================================
- Coverage     72%     72%   -0%     
=====================================
  Files        383     383           
  Lines      62349   62347    -2     
  Branches   62349   62347    -2     
=====================================
- Hits       44637   44632    -5     
+ Misses     15377   15376    -1     
- Partials    2335    2339    +4     

Files	Coverage Δ
engine/src/witness/btc.rs	27% <0%> (-<1%)	⬇️
engine/src/witness/eth.rs	0% <0%> (ø)
engine/src/witness/dot.rs	29% <0%> (+1%)	⬆️
engine/src/witness/start.rs	0% <0%> (ø)

... and 3 files with indirect coverage changes

📣 Codecov offers a browser extension for seamless coverage viewing on GitHub. Try it in Chrome or Firefox today!

[kylezs]

The only thing that's still unclear to me is whether we also want to specialise is_header_ready

I think I'd rather leave this for now until/if there's a need for it. This gets us most of the way there, and introducing that might increase the complexity a bit, when we could be deleting pre-witnessing in favour of something more sophisticated for instant-ingress.