Add a JoinDataSource

[doekenorg]

It would be cool if we could join multiple DataSources on each other, based on a shared value.

Some things to consider:

1. Duplicate field names should be resolvable by adding the data source id (or an alias) in front of the field (a.field_name)
2. A DataSource needs to support joining (introduce an interface). To avoid the n+1 problem when joining tables it would be helpful if data sources could retrieve data based on the join data. This way it should be able to retrieve the correct amount of data with the limit and offset.
3. Sorting and filtering will be challenging if it happens on multiple data sources at a time. Getting the limit and offset right might not even be possible.

In order for this to be implemented; we need real life examples to reproduce.