[SPARK-17900][SQL] Graduate a list of Spark SQL APIs to stable

[rxin]

What changes were proposed in this pull request?

This patch graduates a list of Spark SQL APIs and mark them stable.

The following are marked 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

The following are kept 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

We should discuss more whether we want to mark Dataset typed operations stable in 2.1.

How was this patch tested?

N/A - just annotation changes.

[SparkQA]

Test build #66910 has finished for PR 15469 at commit f712dd4.

• This patch fails MiMa tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #3338 has finished for PR 15469 at commit f712dd4.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[rxin]

cc @marmbrus

[marmbrus]

LGTM, merging to master.