Add missing benchmarks.

[shamil-gadelshin]

Description

This PR adds missing benchmarks for staking extrinsics:

• benchmark_add_stake_limit
• benchmark_remove_stake_limit
• benchmark_add_stake_limit_aggregate
• benchmark_remove_stake_limit_aggregate

This is a subtask for #1525

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Other (please describe): add benchmarks for existing code

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have run cargo fmt and cargo clippy to ensure my code is formatted and linted correctly
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules