Add the capability to read items from a file

[fcsonline]

Add the capability to read items from a file

Nowadays, it is really difficult to configure this GitHub action, if you have many events, logs, metrics, check to push to DataDog. On the other hand, if the list of items is generated dynamically, then you have to fight to generate a valid yaml string.

This commit adds the capability to configure a source yaml file to read the items from.