BN<>VC interop: Export the active preset to fill missing preset for spec api

[g11tech]

Motivation
This PR addresses lodestar validator interop with the other clients (LH)

• As now merge related fields are present in the preset, this PR also picks the active preset to fill for missing fields in the interop spec api.
• Tested that no additional params need to be specified in the --paramsFile, since default activiation of merge (and other forks like sharding) via MERGE_EPOCH is set to far future and hence shouldn't interfere with the operation of validator if the spec api don't have those fields. This addresses the need to have a better UX as desired in Canot start validator due to INACTIVITY_PENALTY_QUOTIENT_MERGE being undefined #3437

Tested against Lighthouse BN ✔️

Closes #3437

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[codecov]

Codecov Report

Merging #3574 (07d1834) into master (659e3e1) will decrease coverage by 0.01%.
The diff coverage is n/a.

❗ Current head 07d1834 differs from pull request most recent head 0f675e1. Consider uploading reports for the commit 0f675e1 to get more accurate results

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