Batcher: simplify ChannelOut initialization

[mdehoog]

Description

Simplify the initialization of new derive.ChannelOut implementations in the batcher. Includes:

• Removing genesisTimestamp and chainID from NewSpanChannelOut (they are instead exposed from the rollup.ChainSpec struct
• Move ChannelOut selection to a separate function for clarity
• Changed the rollup.Config variable to a reference in ChannelBuilder, to avoid the unnecessary referencing/dereferencing.

Tests

There are no logic changes, apart from the ChannelBuilder.rollupCfg, which should be covered by existing tests.

Additional context

Paving the way for customization of the derive.ChannelOut, which will come in a follow up.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.87%. Comparing base (0543bf7) to head (e98b4ca).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #12045      +/-   ##
===========================================
- Coverage    79.10%   78.87%   -0.24%     
===========================================
  Files           41       41              
  Lines         3437     3437              
===========================================
- Hits          2719     2711       -8     
- Misses         548      557       +9     
+ Partials       170      169       -1     

Flag	Coverage Δ
cannon-go-tests	78.87% <ø> (-0.24%)	⬇️

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see 1 file with indirect coverage changes