Refactor optimization statistics serialization

[jhonabreul]

Description

Refactor optimization statistics serialization to support custom statistics.

Related Issue

Motivation and Context

Requires Documentation Change

How Has This Been Tested?

Unit tests

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)
• [x] 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>