Adding missing datasource attribute to mapfish-print externs

[pfirpfel]

see:
https://mapfish.github.io/mapfish-print-doc/attributes.html#!datasource

[fredj]

@sbrunner is {Array} correct? or can we have an other type for the datasource ?

[fredj]

if the type is free, we should use {?} meaning "Indicates that the variable can take on any type, and the compiler should not type-check any uses of it."

see https://github.com/google/closure-compiler/wiki/Type-Expressions#the-unknown-type

[fredj]

@pfirpfel According to @sbrunner ; the type should be Array.<Object>

[fredj]

@pfirpfel thanks, please merge