Make input/form control value partial?

[maxime1992]

Making the input (of a root form component) or the value (set on a sub form component) be a partial of the interface would open new possibilities such as:

• No need to pass an object containing all null values but 1 or 2 to set a default
• Less confusing for people: We often get the question of "why do I need to pass an exact match of the form?"
• should createFormArrayControl accept null values for the value object #118 Wouldn't be an issue anymore

This might be a big change but could change the architecture quite a bit so limit to a spike