Add option to generate outbound scid alias using intercept namespace

[aylagreystone]

Introduces a flag on ChannelHandshakeConfig that can be set to enable outbound scid alias generation to use the intercept namespace. This would allow users to selectively enable interception on channels and get HtlcIntercepted events for every payment forwarded on those channels.

[ldk-reviews-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 82.60870% with 4 lines in your changes missing coverage. Please review.

Project coverage is 89.84%. Comparing base (0848e7a) to head (363cf07).
Report is 57 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/util/config.rs	25.00%	1 Missing and 2 partials ⚠️
lightning/src/ln/channelmanager.rs	94.44%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3839      +/-   ##
==========================================
- Coverage   89.88%   89.84%   -0.04%     
==========================================
  Files         160      160              
  Lines      129654   129670      +16     
  Branches   129654   129670      +16     
==========================================
- Hits       116534   116507      -27     
- Misses      10425    10458      +33     
- Partials     2695     2705      +10     

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[tnull]

As mentioned on Discord, I don't think we should extend the intercept SCID concept much further. If we want to do generalized HTLC interception (#2855), we should finally do it 'properly', i.e., create an interface (e.g., a trait) that would allow users to intercept all HLTCs and then return an action to be taken. This could for example be similar to what CLN does with its hooks.

[tnull]

Going ahead and closing this as it has been superseded by the draft at #3843.