Remove crontab-exec dependency

For docker environments, it would not be common to run a cron in the same container as the main application process.

For those or similar use cases it would be great if flask-crontab could just output the crontab contents so it could be re-used with any other similar cron-daemon.

Start by locating the command or entry point that currently invokes crontab-exec and inspect how scheduled tasks are assembled. Define an output mode that emits the crontab contents for reuse by another cron daemon, then verify that the existing in-container behavior is not broken.

Written by the indexing model from the issue text.