[knob-framework] Knob Framework V2

[angela28chen]

Adding a newer version of the knob framework and a demo that utilizes it.

Added in this PR:

• ApplicationSettings.useKnobManagerNew : A flag for applications to use the new knob framework on top of the old one
• OptionsNew: An intermediate format along with some supporting classes (JsonConverterNew and CommandLineParserNew) that are intended to replace CliOptions and CommandLineParser
• KnobManagerNew and KnobNew
• knob_demo/ : Example project that features the newer knob framework
• Minor changes to string_util, adding Split function and removing whitespace padding from compound strings produced by ToString() functions

Some notable features of the newer version of knob framework:

• Decoupled knob function and display
• Runtime adjustable knob parameters (e.g. min, max, available subset of options)
• Runtime Load/Save support with JSON config files
• Can now identify if non-knob options are specified on the command line and terminate the application early

Future plans:

• Switch all projects and benchmarks to use the new knob framework.
• Change ppx::Application to use only the new knob framework and mStandardOptsNew
• Remove code for old knob framework and CommandLineParser + CliOptions
• Rename the replacements to remove "New" suffix