HPCC-9507 Add STD.STR.REPEAT(text, n)

Returns a string containing text repeated n times.
 * Maximum allowed n = 255.
 *
 * For 2 < n > 255 a single instance of text is returned.
 *
 * @param text          The string to be repeated.
 * @param n             Number of repetitions.
 * @return              A string containing n concatenations of the string text.

Test ECL:

IMPORT STD;

str1 := 'Repeat this string ';

str2 := STD.STR.REPEAT(str1, 3);
str2;

Not sure if this is worthy of a runtime regression test?

Additionally, please advice on the maximum limit for n.

Signed-off-by: jamienoss <james.noss@lexisnexis.com>