Spark 3.0 readiness part 2

[Niharikadutta]

This PR adds support for following APIs added in Spark 3.0:

Functions APIs

• scala/python def xxhash64(cols: Column): Column
• scala def split(str: Column, pattern: String, limit: Int): Column
• scala/python def overlay(src: Column, replace: Column, pos: Column, len: Column): Column
• scala def overlay(src: Column, replace: Column, pos: Column): Column
• scala def add_months(startDate: Column, numMonths: Column): Column
• scala def date_add(start: Column, days: Column): Column
• scala def date_sub(start: Column, days: Column): Column
• scala def schema_of_json(json: Column, options: java.util.Map[String, String]): Column
• scala/python def map_entries(e: Column): Column
• scala/python def from_csv(e: Column, schema: StructType, options: Map[String, String]): Column
• scala def from_csv(e: Column, schema: Column, options: java.util.Map[String, String]): Column
• scala def schema_of_csv(csv: String): Column
• scala def schema_of_csv(csv: Column): Column
• scala/python def schema_of_csv(csv: Column, options: java.util.Map[String, String]): Column
• scala/python def to_csv(e: Column, options: java.util.Map[String, String]): Column
• scala def to_csv(e: Column): Column
• scala def years(e: Column): Column
• scala def months(e: Column): Column
• scala def days(e: Column): Column
• scala def hours(e: Column): Column
• scala def bucket(numBuckets: Column, e: Column): Column
• scala def bucket(numBuckets: Int, e: Column): Column

This PR addresses part of #633

[imback82]

LGTM except for one nit comment, thanks @Niharikadutta!

[suhsteve]

LGTM.