Add benchmark module

[minwoox]

so that we can run a benchmark to check whether a change improves or reduce performance
Example: https://github.com/line/armeria/blob/2caa0f8812e7435b7141136da95b5fd1f3efa2be/benchmarks/jmh/README.md

EDIT:
I realized that we set up benchmark only for the server module.

centraldogma/server/src/jmh/java/com/linecorp/centraldogma/server/internal/storage/repository/git/GitRepositoryBenchmark.java

Line 42 in 2155e5a

public class GitRepositoryBenchmark {

We need to add a dedicate module for benchmark so that other modules besides server can use it.