op-e2e: Separate engine API implementation from backend

[ajsutton]

Description

Separate the op-e2e engine API implementation from the backend. The engine API will be used in the fault proof program as well with an oracle based backend.

The engine api implementation has moved to its own module since it will now be production code, not just part of the e2e infrastructure.

Tests

Introduced the l2_engine_api_tests.go file which provides a set of tests using a pluggable backend. This tests the engine API code better but also allows testing compatibility with each engine backend, reusing the same tests. Currently the op-e2e backend is all that calls these tests.

Metadata

• Fixes https://linear.app/optimism/issue/CLI-3740/l2-execution-engine-api

TODOs

• Author or reviewer has added an entry to the current release notes draft, if appropriate.

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[codecov]

Codecov Report

Merging #5335 (36c25d3) into develop (b424c7c) will decrease coverage by 4.37%.
The diff coverage is 0.00%.

❗ Current head 36c25d3 differs from pull request most recent head 773f99b. Consider uploading reports for the commit 773f99b to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5335      +/-   ##
===========================================
- Coverage    39.88%   35.52%   -4.37%     
===========================================
  Files          391      238     -153     
  Lines        25657    21614    -4043     
  Branches       838        0     -838     
===========================================
- Hits         10233     7678    -2555     
+ Misses       14644    13194    -1450     
+ Partials       780      742      -38     

Flag	Coverage Δ
bedrock-go-tests	35.52% <0.00%> (-0.86%)	⬇️
common-ts-tests	?
contracts-bedrock-tests	?
contracts-tests	?
core-utils-tests	?
dtl-tests	?
fault-detector-tests	?
sdk-tests	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
op-program/l2/engineapi/l2_engine_api.go	0.00% <0.00%> (ø)
...p-program/l2/engineapi/test/l2_engine_api_tests.go	0.00% <0.00%> (ø)

... and 162 files with indirect coverage changes

[protolambda]

LGTM, nice cleanup. Regarding non-L2 usage: I think it's a good idea, but also scope-creep, and we can refactor that to be more general in a separate PR.

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[mergify]

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