[FEA] Create Java string scalar from byte array

[jlowe]

Is your feature request related to a problem? Please describe.
In Spark, strings are tracked with a UTF8String which is essentially a wrapper around a byte array. The Java Scalar only allows creating string scalars by passing a Java String, and creating that from a byte array requires copying the bytes to a String. Internally Scalar.fromString copies the String data to a byte array so it can be copied to the GPU.
Therefore when converting a Spark UTF8String into a Scalar there's two extra copies that are created that should be avoidable.

Describe the solution you'd like
Scalar should have a fromBytesAsString method that can take a byte array and convert it into a GPU string scalar.

[github-actions]

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[jlowe]

Fixed by #8294