This also unifies `get_default_value` helper across all the target
languages.
Before this, there were two different versions of it for python
and typescript while c++ side didn't have one at all.
After this commit, every non-key field is treated as if it was optional
while every key field (plus every auto-generated `Kind` field) - as if
it was required.
This allows to avoid having to specify the `rootType` (stable within
the entire tree) manually each time a reference is introduced.
Thanks to that, a much nicer syntax of:
```yaml
- name: MyTypeField
reference-to: TypeDefinition
```
can replace the current one:
```yaml
- name: MyTypeField
reference:
pointeeType: TypeDefinition
rootType: Binary
```
Ivan Krysak