HPCC-12564 New regressionsuite option to generate multiple variants

Update after review:
  - remove -dymanic command line parameter and handler
  - remove //dynamic tag handler
  - remove dynamicsource_tag.ecl and xml
  - remove issue12565.ecl and xml
  - update README.rst

Signed-off-by: Attila Vamos <attila.vamos@gmail.com>