[FEA] Enable StringLPad

[sameerz]

Is your feature request related to a problem? Please describe.
Support https://spark.apache.org/docs/latest/api/sql/#lpad . CUDF provides a pad function: https://docs.rapids.ai/api/cudf/stable/api.html#cudf.core.column.string.StringMethods.pad

Describe the solution you'd like
Allow Spark SQL queries to use the lpad function on strings.

[revans2]

To make this work we are going to have to put in JNI bindings for cudf::strings::pad_side, cudf::strings::pad, and cudf::strings::slice_strings so we can match the truncate that spark does for strings that are too long.