alter: extract opts_parse_key from opts_create_from_field

In the next patches the field_def will being parsed from space:format.
Field_def will contain char *name, which is limited by BOX_NAME_MAX = 65000.
So neither opt_type OPT_STR or OPT_STRPTR can be used to parse this name from
space:format. Besides, field_def contains enum field_type, which can not
be parsed using and opt_type.
Also, field_def will contain default_value, which can store values of
many types.

Proposal is to use opt_create_from_field not for entire field_def, but only
for several fields using opts_parse_key. And parse other options
manualy.