Add FlatListProof#verify benchmark and make it public

[dmitry-timofeev]

Overview

h=10:

Benchmark                                                       (height)   Mode  Cnt      Score     Error   Units
FlatListProofBenchmark.verify                                         10  thrpt   10     87,436 ±   0,961  ops/ms

More experiments can be run with: java -jar target/benchmarks.jar -f 1 -wi 5 -w 1s -i 10 -r 2s -p height=<comma-separated heights> verify

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See: https://jira.bf.local/browse/ECR-XYZ

Definition of Done

• There are no TODOs left in the code
• Change is covered by automated tests
• The coding guidelines are followed
• Public API has Javadoc
• Method preconditions are checked and documented in the Javadoc of the method
• Changelog is updated if needed (in case of notable or breaking changes)
• The continuous integration build passes