Enhance FieldDefinition provision - investigate DefaultFormula property support

[SubPointSupport]

NumberFieldDefinition can be set to display default value as "formula".

Seems to be DefaultFormula property on the SPFIeld.

Is not available with the CSOM (maybe we can hit it with the firs provision constructing field's XML), and can be adjusted with OnProvisioned event on the real field instance. A further investigation is needed.

[SubPointSupport]

SPField.DefaultFomula should be used for SSOM.
CSOM does not support that property, initial XML provision should be used.

Must be covered by the regression tests.

[SubPointSupport]

Added FieldDefinition.DefaultFormula support for SSOM as straight mapping to SPField.DefaultFormula field. Not supported in CSOM yet, a separate task is created:

#842