Implement reading and setting default values

[duskybomb]

Reading and set default if value is not defined

• What all fields can have _default?
• Should I add as many fields as possible to fields.yml?

[m3nu]

Tests and actual default values?

[duskybomb]

For tests, we need to resolve issue with _required fields (whether name and LineTotalAmount are required fields or not), because else is_valid() will always return False^[1]. It is also affecting other features and tests as well.

[duskybomb]

What all fields can have default values?