Suppress Equal Diff on Schedule Layer Timestamps

[stmcallister]

This change allows the provider to treat UTC values for the start, rotation_virtual_start, and end fields on pagerduty_schedule.layer as equal to the equivalent offset returned by the PagerDuty API. This comes in handy for handling daylight savings. This change also validates that the timestamps are the proper format as well.

Related issue: #200

Test output on schedule object

 TF_ACC=1 go test -run "TestAccPagerDutySchedule" ./pagerduty -v -timeout 120m 
=== RUN   TestAccPagerDutySchedule_import
--- PASS: TestAccPagerDutySchedule_import (7.16s)
=== RUN   TestAccPagerDutySchedule_Basic
--- PASS: TestAccPagerDutySchedule_Basic (8.21s)
=== RUN   TestAccPagerDutyScheduleOverflow_Basic
--- PASS: TestAccPagerDutyScheduleOverflow_Basic (7.58s)
=== RUN   TestAccPagerDutySchedule_BasicWeek
--- PASS: TestAccPagerDutySchedule_BasicWeek (7.56s)
=== RUN   TestAccPagerDutySchedule_Multi
--- PASS: TestAccPagerDutySchedule_Multi (9.22s)
PASS
ok  	github.com/terraform-providers/terraform-provider-pagerduty/pagerduty	48.751s

[heimweh]

Nice work @stmcallister 👌 🙂