Code Review followup: Use Optional<> and create a new Map for length

Created new class PhoneNumberRequiredLengths.java with a Map lookup for the
requiredlength of phone numbers in select countries that are well-known to
have uniform fixed-length phone number throughout. Country codes missing
will continue to use just the previous length-range algorithms.  Lookup
into this Map is integrated into PhoneNumberValidator.java, no longer
needed an overloaded constructor to provide this functionality.
[New .java file was accidentally omitted from previous commit]

desktop/src/main/java/bisq/desktop/util/validation/PhoneNumberRequiredLengths.java

@@ -0,0 +1,47 @@
+package bisq.desktop.util.validation;
+
+import java.util.Map;
+
+import static java.util.Map.entry;
+
+final class PhoneNumberRequiredLengths {
+    /**
+     * Immutable mapping of ISO 3166 alpha-2 country code to the required length of the phone number (in digits, exluding country code prefix).
+     *
+     * Used by PhoneNumberValidator.java 
+     * If an entry is not present in the map, the requiredLength option will not be used in the validation.
+     */
+    private final static Map<String, Integer> NUMBER_LENGTH_MAP = Map.ofEntries(
+            entry("AG", 7 ),  // CountryCode: "1-268"
+            entry("AI", 7 ),  // CountryCode: "1-264"
+            entry("AS", 7 ),  // CountryCode: "1-684"
+            entry("BB", 7 ),  // CountryCode: "1-246",
+            entry("BM", 7 ),  // CountryCode: "1-441"
+            entry("BS", 7 ),  // CountryCode: "1-242"
+            entry("CA", 10),  // CountryCode: "1"
+            entry("DM", 7 ),  // CountryCode: "1-767"
+            entry("DO", 10),  // CountryCode: "1"     (DO has three area codes 809,829,849; let user define hers)
+            entry("GD", 7 ),  // CountryCode: "1-473"
+            entry("GU", 7 ),  // CountryCode: "1-671"
+            entry("JM", 7 ),  // CountryCode: "1-876"
+            entry("KN", 7 ),  // CountryCode: "1-869"
+            entry("KY", 7 ),  // CountryCode: "1-345"
+            entry("KZ", 10),  // CountryCode: "7"
+            entry("LC", 7 ),  // CountryCode: "1-758"
+            entry("MP", 7 ),  // CountryCode: "1-670"
+            entry("MS", 7 ),  // CountryCode: "1-664"
+            entry("PR", 10),  // CountryCode: "1"
+            entry("RU", 10),  // CountryCode: "7"
+            entry("SX", 7 ),  // CountryCode: "1-721"
+            entry("TC", 7 ),  // CountryCode: "1-649"
+            entry("TT", 7 ),  // CountryCode: "1-868"
+            entry("US", 10),  // CountryCode: "1"
+            entry("VC", 7 ),  // CountryCode: "1-784"
+            entry("VG", 7 ),  // CountryCode: "1-284"
+            entry("VI", 7 )   // CountryCode: "1-340"
+    );
+
+    static Integer getRequiredLength(String isoCountryCode) {
+        return NUMBER_LENGTH_MAP.get(isoCountryCode);
+    }
+}