schema: allow to store custom fields in format's field definition

Some users store in format fields their custom keys. But current
opts parser does not allow to store any unknown keys. Lets allow it.

Example:
format = {}
format[1] = {name = 'field1', type = 'unsigned', custom_field = 'custom_value'}
s = box.schema.create_space('test', {format = format})
s:format()[1].custom_field == 'custom_value'

Closes #2839