[retried_tests.ml]: refactoring + can get retried tests of given pipelines

By default, the metrics retried_tests.ml fetches all flaky tests since a given date. However, for debugging is often convenient to instruct it to scan a given, or a set of, pipeline(s).

This MR adds the Tezt argument retried_tests.pipelines that enables just this: it takes a list of pipeline ids and it restricts scanning to those pipelines.

Manual test

To test:

$ GL_ACCESS_TOKEN=glpat-... TZ=UTC dune exec bin/main.exe -- -f retried_tests.ml -i -a retried_tests.pipelines=1038070553,1038070553