[INGEST][DATASTORE][API] Allow for an optional list of alternative WIGOS Station Identifiers

[jo-asplin-met-no]

In addition to the WIGOS Station Identifier (WSI) kept in the platform metadata field (a string), it would be useful to keep a list of secondary/alternative WSIs in an alt_platforms field (an array of strings).

Attributes like country codes and (old) WMO station numbers can be encoded into alternative WSI and allow for quick selection.