feat: Add [DOMAIN_EXPIRATION] placeholder for monitoring domain expiration using WHOIS (#325)

* feat: Add [DOMAIN_EXPIRATION] placeholder for monitoring domain expiration using WHOIS

* test: Fix issue caused by possibility of millisecond elapsed during previous tests

* test: Fix test with different behavior based on architecture

* docs: Revert accidental change to starttls example

* docs: Fix mistake in comment for Condition.hasIPPlaceholder()

core/result.go

@@ -41,6 +41,9 @@ type Result struct {
 	// CertificateExpiration is the duration before the certificate expires
 	CertificateExpiration time.Duration `json:"-"`
 
+	// DomainExpiration is the duration before the domain expires
+	DomainExpiration time.Duration `json:"-"`
+
 	// body is the response body
 	//
 	// Note that this variable is only used during the evaluation of an Endpoint's health.