[CSGI-2017 ] Update pagerduty_user_contact_method address validation logic

[imjaroiswebdev]

Update format validation for pagerduty_user_contact_method.address based on PagerDuty officially supported phone number formatting rules.

Introduces the following new test cases...

$ make testacc TESTARGS="-count=1 -run TestAccPagerDutyUserContactMethodPhone_FormatValidation"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -count=1 -run TestAccPagerDutyUserContactMethodPhone_FormatValidation -timeout 120m
?       github.com/terraform-providers/terraform-provider-pagerduty     [no test files]
=== RUN   TestAccPagerDutyUserContactMethodPhone_FormatValidation
--- PASS: TestAccPagerDutyUserContactMethodPhone_FormatValidation (1.72s)
PASS
ok      github.com/terraform-providers/terraform-provider-pagerduty/pagerduty   2.428s