Rework default base64 implementation

[josesimoes]

Description

• Now using mbedTLS version as default and for Win32 CLR.

Motivation and Context

• The simplified default implementation was failing on some edge cases. Better play safe and use a know good one.

How Has This Been Tested?

Screenshots

Types of changes

• Improvement (non-breaking change that improves a feature, code or algorithm)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Config and build (change in the configuration and build system, has no impact on code or features)
• Dependencies (update dependencies and changes associated, has no impact on code or features)
• Unit Tests (work on Unit Tests, has no impact on code or features)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[nfbot]

@josesimoes there are issues with the code style on the source files.
A PR was submitted with the code style fixes. Please click Eclo#86, review the changes if you want and merge it.

Make sure to follow the project code style. Check the details here on how it works and the tools required to help you with that.