feat(sequencer): Origin Selector asynchronously prefetches the next origin from events

[BrianBland]

Description
Updates the Sequencer's L1 origin selector to locally cache the currentOrigin and nextOrigin attributes and also optimistically prefetch the nextOrigin whenever an ForkchoiceUpdateEvent event is received. This is expected to remove synchronous origin selection from the critical path of sequencer block production, although the impact of this is reduced when using a single-threaded event loop. (Addressed by #11100)

In many cases, the existing L1 origin is "good enough" for building a new block from the sequencer's perspective, and it's better to fetch the next L1 origin in the background instead of spending precious time that could otherwise be used for building the next block. Whenever the nextOrigin is already available and valid, this is returned instead.

Under specific circumstances, such as when a new L1 origin has not been found for a long period of time, the sequencer will actually need to review the nextOrigin and determine if this is valid (behind the next L2 block time). In these cases, the FindL1Origin call will fetch the next possible origin synchronously, and compare these potential origins immediately.

Note: This is a simplified version of #12073, which uses no goroutines and fully embraces the async event model.

Tests

Added several new origin selector tests to ensure that this new prefetching behavior is safe under all conditions.

Metadata

Partially addresses #11960. In order to fully resolve this ticket, we may need to merge the origin selector and attributes builder, so that an origin is only selected if and only if it also has the relevant attributes already built or has all of the data needed to build attributes (L1 receipts).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.97%. Comparing base (d141b53) to head (bec9b29).
Report is 36 commits behind head on develop.

Additional details and impacted files

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  Coverage    74.97%   74.97%           
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  Files           49       49           
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  Hits          2741     2741           
  Misses         743      743           
  Partials       172      172           

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[ajsutton]

LGTM. Sorry for the delay in reviewing.