Allow cron syntax for specifying polling cycle for LdioHttpInPoller

[rorlic]

Is your feature request related to a problem? Please describe.
Currently the polling interval of the LdioHttpInPoller can be defined in ISO8601 duration format, resulting in the polling happen on start and then every X amount of time as defined by the duration. However, sometimes a Data Publisher needs to have more control over when the polling actually happens, e.g. at 3:15 AM daily. Unless you launch the system at that particular moment, it is currently not possible to achieve this.

Describe the solution you'd like
In addition to the current interval configuration, please provide a config setting accepting a cron specification, mutually exclusive.
Deprecate the interval specification with a logged warning and remove the interval config option at some later time.
OR (less preferable)
Replace the interval config by the cron config and create a new major version (2.0) for LDIO, breaking backwards compatibility.

[jeflaenen]

Jira ticket https://vlaamseoverheid.atlassian.net/browse/VSDSPUB-1105 is created for this.