Speed up StringUtils.getDigits(String)

Local results:
```
Benchmark                                               (length)  Mode  Cnt      Score     Error  Units
StringUtilsGetDigitsBenchmark.testGetDigitsManually           10  avgt   25     19.835 ±   0.321  ns/op
StringUtilsGetDigitsBenchmark.testGetDigitsManually          100  avgt   25     91.149 ±   1.234  ns/op
StringUtilsGetDigitsBenchmark.testGetDigitsManually         1000  avgt   25    777.029 ±   9.725  ns/op
StringUtilsGetDigitsBenchmark.testGetDigitsManually        10000  avgt   25   7883.407 ±  61.312  ns/op
StringUtilsGetDigitsBenchmark.testGetDigitsWithBuilder        10  avgt   25     84.027 ±  36.391  ns/op
StringUtilsGetDigitsBenchmark.testGetDigitsWithBuilder       100  avgt   25    334.174 ± 225.501  ns/op
StringUtilsGetDigitsBenchmark.testGetDigitsWithBuilder      1000  avgt   25   1700.380 ±   2.512  ns/op
StringUtilsGetDigitsBenchmark.testGetDigitsWithBuilder     10000  avgt   25  16191.400 ±  18.637  ns/op
```

Author: yuanchaodu

src/main/java/org/apache/commons/lang3/StringUtils.java

@@ -2043,15 +2043,17 @@ public static String getDigits(final String str) {
         if (isEmpty(str)) {
             return str;
         }
-        final int sz = str.length();
-        final StringBuilder strDigits = new StringBuilder(sz);
-        for (int i = 0; i < sz; i++) {
+        final int len = str.length();
+        final char[] buffer = new char[len];
+        int count = 0;
+
+        for (int i = 0; i < len; i++) {
             final char tempChar = str.charAt(i);
             if (Character.isDigit(tempChar)) {
-                strDigits.append(tempChar);
+                buffer[count++] = tempChar;
             }
         }
-        return strDigits.toString();
+        return new String(buffer, 0, count);
     }
 
     /**