Handling several indexes per datasource

Currently, a quickwit datasource requires to define an index.

We want a user to be able to query different indexes (using the index pattern) or query multiple indexes with a single datasource.

Here is the proposed spec.

Datasource parameters

• timestamp field(mandatory): This field is used across all views and ensures that all indexes used are on the same timestamp field. We could make it optional but we currently don't support querying multiple indexes with different timestamp fields.
• timestamp output format (mandatory): we don't want the user to set this value, but I think we don't really have the choice for now. This format is used by the Grafana plugin to show correctly the datetime. We could guess the format at the backend plugin level though.
• default index(optional): either an index ID or an index pattern or empty.

Query builder

We should add an index field filled with the default value in the query builder.

Validation

If an index or index pattern is defined, we can validate the timestamp field and timestamp field output format.