Update-check feature

the old assistant had it:
> ``update-check``: how often ``daml`` will check for new versions of the SDK, in seconds (default to ``86400``, i.e. once a day)

The issue names no files, tests, or entry points; start by locating the dpm CLI's version-check or update-related handling. Implement an update-check setting measured in seconds, using 86400 seconds as the default, and verify that the package manager checks for new SDK versions at that interval.

Written by the indexing model from the issue text.