Move benchmarks from main typeberry repository

## Context

As discussed in https://github.com/FluffyLabs/typeberry/pull/690#discussion_r2401391644, we should move the benchmarks from the main typeberry repository to this testing repository.

## Motivation

- Separates testing/benchmarking dependencies from the main codebase
- Allows keeping benchmark-specific dependencies (e.g., `blake2b` for comparison) without adding them to the main repository
- Cleaner separation of concerns

## Tasks

- [ ] Move benchmark files from `benchmarks/` directory in typeberry
- [ ] Set up benchmark infrastructure in typeberry-testing
- [ ] Update CI/CD to run benchmarks from the new location
- [ ] Document how to run benchmarks in the new repository

## References

- PR: https://github.com/FluffyLabs/typeberry/pull/690
- Discussion: https://github.com/FluffyLabs/typeberry/pull/690#discussion_r2401391644
- Requested by: @tomusdrw