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

## 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.

Author: Reynold Xin <rxin@databricks.com>

Closes #15469 from rxin/SPARK-17900.