[SPARK-3572] [sql] [mllib]  User-Defined Types and MLlib Datasets

This PR adds User-Defined Types (UDTs) to SQL.  It is a precursor to using SchemaRDD as a Dataset for the new MLlib API.

## Main additions

Public API
* SQL
 * Added annotation SQLUserDefinedType (DeveloperApi)
 * Added UDTRegistry (global object)
 * Added abstract class UserDefinedType
* MLlib
 * Vector, DenseVector, SparseVector are annotated with SQLUserDefinedType

Internals
* Made MLlib depend on SparkSQL.
* SQL
 * ScalaReflection
  * Methods for converting between Scala and Catalyst types now take DataType.
   * convertRowToScala added in several locations in SQL
  * schemaFor checks for SQLUserDefinedType annotation and checks UDTRegistry
* MLlib
 * Added VectorUDT, DenseVectorUDT, SparseVectorUDT (private[spark])

Examples
* /examples/mllib/DatasetExample.scala: Demonstrates implicit conversion of RDD[LabeledPoint] to SchemaRDD

Unit Tests
* mllib/rdd/DatasetSuite.scala: Tests *VectorUDT
* sql/UserDefinedTypeSuite.scala: Tests fake version of DenseVector

## Design decisions

* UDTs override types natively recognized by SQL.

* Question: Should users be able to override primitive or built-in types?

## Items left for future PRs

* Java and Python APIs
* Serialization (Parquet, etc.)

CC: @mengxr @marmbrus
Can this have some extra documentation about what it's purpose is and when a user might want to define one?