It will fix the proxy parse issue in case user update the cert on
windows and add blank line at the end of certificate. Unit test is also
added for same.

When the cert file ends with \r\n, trimTrailingEOL would only remove \n
and the file will end with \r. Then this causes issues in regexp which
only replace `\r\n` => `\n` but doesn't perform any action for `\r` and
eventualy turn up this bug.

fixes: crc-org#3785

Looks like we are using `strings.TrimRight(<string>, "\n\r")` at
multiple locations so better to move this helper to pkg/strings for
better maintainance.