Accepts string value for query param "fields"

[ezra-obiwale]

In addition to accepting GET param fields like

fields[table1]=col1,col2&fields[table2]=col1,col2

this PR makes it possible to accept values like

fields=table1.col1,table1.col2,table2.col1,table2.col2

And if table1 is the table for the target resource, then it can simply be like

fields=col1,col2,table2.col1,table2.col2

[BBrey]

We would really love this, because now we actually need to fill in the table name like table_link_to_table when using the fields

[AlexVanderbist]

Nice PR, thanks!