Refactor ApiClient to use HttpClient

[jhonabreul]

Description

Refactor ApiClient to use HttpClient. This is part of #8603 efforts to remove RestSharp dependency.

The underlying RestSharp client is kept until we can remove all usages of ApiConnection.TryRequest (and other methods) that are thighly coupled to RestSharp since they take RestRequest as a parameter and they are used widely in Lean's codebase.

Related Issue

Part of #8603

Motivation and Context

Requires Documentation Change

How Has This Been Tested?

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] Refactor (non-breaking change which improves implementation)
• [ ] Performance (non-breaking change which improves performance. Please add associated performance test and results)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Non-functional change (xml comments/documentation/etc)

Checklist:

• [x] My code follows the code style of this project.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.
• [x] My branch follows the naming convention bug-<issue#>-<description> or feature-<issue#>-<description>