[Workshop] Add new class SchemaReader to ease the manipulation of alert content.

[JulienPeloton]

Describe the issue
One comment made on the workshop was the difficulty to know from the client what kind of information was available in alerts (especially the distinction between ZTF/LSST original data and Fink added value).

Action item: add a new class SchemaReader that reads the provided schemas and display in a friendly way information about the alert content. Note - this should be generic enough that it could be used by AlertReader (use fastavro routines to decode avsc and parse them correctly).