feat: in code config

[Jakub-Vacek]

This PR enables passing IConfig variables through SuperfaceClient constructor. These values are merged with config loaded from environment. Value passed in code has higher priority. Properties like debug will be added in separate PR.

Description

Motivation and Context

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] I have updated the documentation accordingly. If you are changing code related to user secrets you need to really make sure that security documentation is correct.
• [x] I have read the CONTRIBUTING document.
• [x] I haven't repeated the code. (DRY)
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.