[Ruby] Add ability to read results of min_max compute function

f = Arrow::Function.find('min_max')
f.execute([table['revenue'].data]).value
#<Arrow::StructScalar:x11fcc8df8 ptr=x7ffd0ea762b0 {min:double = 0, max:double = 257.515735}> 
f.execute([table['revenue'].data]).value.value 
# []