Mark the following Spark SQL APIs as stable

Mark the following stable:

Dataset/DataFrame

• functions, since 1.3
• ColumnName, since 1.3
• DataFrameNaFunctions, since 1.3.1
• DataFrameStatFunctions, since 1.4
• UserDefinedFunction, since 1.3
• UserDefinedAggregateFunction, since 1.5
• Window and WindowSpec, since 1.4

Data sources:

• DataSourceRegister, since 1.5
• RelationProvider, since 1.3
• SchemaRelationProvider, since 1.3
• CreatableRelationProvider, since 1.3
• BaseRelation, since 1.3
• TableScan, since 1.3
• PrunedScan, since 1.3
• PrunedFilteredScan, since 1.3
• InsertableRelation, since 1.3

Keep the following experimental / evolving:

Data sources:

• CatalystScan (tied to internal logical plans so it is not stable by definition)

Structured streaming:

• all classes (introduced new in 2.0 and will likely change)

Dataset typed operations (introduced in 1.6 and 2.0 and might change, although probability is low)

• all typed methods on Dataset
• KeyValueGroupedDataset
• o.a.s.sql.expressions.javalang.typed
• o.a.s.sql.expressions.scalalang.typed
• methods that return typed Dataset in SparkSession