Dapps-staking precompile: read staked amount on the contract

[Maar-io]

Pull Request Summary

This functionality is an improvement. It adds possibility to read staked amount on the contract for the user (staker). Until now it was only possible to read if user (staker) is staking in general terms.

Check list

• [ ] contains breaking changes
• [ ] adds new feature
• [x] modifies existing feature (bug fix or improvements)
• [ ] relies on other tasks
• [ ] documentation changes
• [x] tests and/or benchmarks are included
• [ ] changed API client type definition or chain metadata

This pull request makes the following changes:

Adds

• (ex: Add feature A)

Fixes

• (ex: Fix validation function)

Changes

• (ex: Change document B)

To-dos

*Feel free to remove this section if it's not applicable

• [ ] (ex: add user list)