Add support for retrieving the merge request approval state

[Ocrosoft]

Can be used for:

• Checking whether approval rules are overridden
• Retrieving Merge Request-level approval rules
• Getting the approval status of each approval rule

[Ocrosoft]

Update

Fixed the compilation error in NGitLab.Mock, and the unit tests for NGitLab.Mock.Tests have passed.

For NGitLab.Tests, some tests were not executed due to the absence of a Docker environment on the local machine.