Adds support for default values

[schottra]

Fixes flyteorg/flyte#18

This change adds support for default values when populating the Launch form. The default values are read from the selected launch plan and set as the initial value for the state hooks used to handle input interaction.

• Added fromLiteral function to all InputHelper implementations. This function is intended to parse a Core.ILiteral (the type used for default values) into an InputValue which can be used to populate a control in the Launch form
• Added defaultValue static property to InputHelper. For most types, this will be unset/undefined, because the default for an untouched control should be completely empty. For booleans, the default value will be set to false. This is because a toggle can't have an empty state and it causes issues when we send a noneType value for booleans.
• Added logic to handle validation of required values
• Added stories for default and required values
• Added/updated helper functions to allow extraction of primitive values when parsing literals.
• Unit tests for the literal parsing