When receive UNSPECIFIED schedule overlap policy, store as SKIP

[lorensr]

Is your feature request related to a problem? Please describe.

temporal.api.enums.v1.schedule.ScheduleOverlapPolicy.SCHEDULE_OVERLAP_POLICY_UNSPECIFIED

• If server stores UNSPECIFIED and later changes its default, the schedule I created earlier thinking it would be default/SKIP will now behave differently
• Would be nice if SDK never received overlap_policy: UNSPECIFIED from DescribeSchedule

Describe the solution you'd like

When server receives UNSPECIFIED schedule overlap policy on create or update, store it as whatever the current default is (atm SKIP).

Additional context

cc @dnr @bergundy @cretz