op-supernode: Update Chain Container Interfaces

[axelKingsley]

• SafeBlockAtTimestamp is now BlockAtTimestamp which accepts a label. I did this with the anticipation of an UnverifiedSafe label, so once it's supported it can be used.
• SafeDB accessors removed from the ChainContainer. The SafeDB is accessed by OptimisticAt and VerifiedAt, but doesn't need direct exposure right now

I'm going to be stacking up more interface extensions and wanted to start from a clean base.

Tests, comments, etc are all updated.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.66%. Comparing base (a7e0935) to head (4c663ae).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop   #18727   +/-   ##
========================================
  Coverage    73.66%   73.66%           
========================================
  Files          190      190           
  Lines        11322    11322           
========================================
  Hits          8340     8340           
  Misses        2838     2838           
  Partials       144      144           

Flag	Coverage Δ
cannon-go-tests-64	66.58% <ø> (ø)
contracts-bedrock-tests	77.57% <ø> (ø)

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