[C++] Construct Array from ExtensionType Scalar

[thisisnic]

Describe the bug, including details regarding any error messages, version, and platform.

As seeing in #45601, in R we get a segfault when we try to create an Expression from an ExtensionType Scalar due to calling MakeArrayFromScalar on an ExtensionType Scalar, which wasn't implemented before.

library(arrow)
x <- 1
class(x) <- "mynewclass"
Expression$scalar(x)

Component(s)

C++

[thisisnic]

Issue resolved by pull request 46675
#46675