feat: simple pre-witnessing

[kylezs]

Review commit by commit.

Pull Request

Closes: PRO-852

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

Introduces simple pre-witnessing. When the prewitness call has been "witnessed" i.e. a super majority have submitted that they've seen it, the SC will emit an event Prewitnessed. This event can then be used by LPs to see incoming swaps.

NB: Because of re-orgs this is inherently unreliable and should not be trusted blindly. It's possible we prewitness something that is not fully witnessed after the lag safety margin.

The CFE simply submits the prewitness extrinsic, when it sees something, without waiting for the safety margin.

[linear]

PRO-852 Pre-Witnessing: Additionally witness deposits after 1 block of confirmation

There should be a new event for this

[codecov]

Codecov Report

Merging #4056 (523603c) into main (406f481) will decrease coverage by 0%.
The diff coverage is 45%.

@@          Coverage Diff           @@
##            main   #4056    +/-   ##
======================================
- Coverage     72%     72%    -0%     
======================================
  Files        374     376     +2     
  Lines      59219   59440   +221     
  Branches   59219   59440   +221     
======================================
+ Hits       42506   42525    +19     
- Misses     14607   14810   +203     
+ Partials    2106    2105     -1     

Files	Coverage Δ
state-chain/pallets/cf-lp/src/lib.rs	69% <ø> (ø)
engine/src/witness/common/chain_source/then.rs	0% <0%> (ø)
...chain_source/chunked_by_vault/deposit_addresses.rs	0% <0%> (ø)
engine/src/witness/dot/dot_source.rs	0% <0%> (ø)
state-chain/pallets/cf-witnesser/src/lib.rs	79% <0%> (-2%)	⬇️
engine/src/witness/start.rs	0% <0%> (ø)
engine/src/witness/dot.rs	27% <3%> (-22%)	⬇️
engine/src/witness/btc.rs	29% <0%> (-33%)	⬇️
engine/src/witness/btc/btc_deposits.rs	73% <73%> (ø)
engine/src/witness/dot/dot_deposits.rs	62% <62%> (ø)
... and 1 more

... and 4 files with indirect coverage changes

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