[Go] Support for casting to extension types

[hermanschaaf]

Describe the enhancement requested

Right now it seems like there is support for casting from extension types to their underlying storage types, but not vice versa.

The cast_test.go tests include these cases, for example:

canCast(types.NewSmallintType(), []arrow.DataType{arrow.PrimitiveTypes.Int16})
canCast(types.NewSmallintType(), numericTypes)                                 // any cast which is valid for storage is supported
canCast(arrow.Null, []arrow.DataType{types.NewSmallintType()})

But adding this case, from the storage type to the extension type, fails:

canCast(arrow.PrimitiveTypes.Int16, []arrow.DataType{types.NewSmallintType()}) // casting from storage type to extension is supported

=== RUN   TestCasts/TestCanCast
    cast_test.go:335: 
        	Error Trace:	/Users/hermanschaaf/code/cloudquery/arrow/go/arrow/compute/cast_test.go:335
        	            				/Users/hermanschaaf/code/cloudquery/arrow/go/arrow/compute/cast_test.go:341
        	            				/Users/hermanschaaf/code/cloudquery/arrow/go/arrow/compute/cast_test.go:401
        	Error:      	Not equal: 
        	            	expected: true
        	            	actual  : false
        	Test:       	TestCasts/TestCanCast
        	Messages:   	CanCast from: int16, to: extension_type<storage=int16>, expected: true

It seems like this should be possible, as long as the type being cast from matches the storage type. Is there another way to do this that I might be missing?

Component(s)

Go