[SPARK-17351] Refactor JDBCRDD to expose ResultSet -> Seq[Row] utility methods

This patch refactors the internals of the JDBC data source in order to allow some of its code to be re-used in an automated comparison testing harness. Here are the key changes:

- Move the JDBC `ResultSetMetadata` to `StructType` conversion logic from `JDBCRDD.resolveTable()` to the `JdbcUtils` object (as a new `getSchema(ResultSet, JdbcDialect)` method), allowing it to be applied on `ResultSet`s that are created elsewhere.
- Move the `ResultSet` to `InternalRow` conversion methods from `JDBCRDD` to `JdbcUtils`:
  - It makes sense to move the `JDBCValueGetter` type and `makeGetter` functions here given that their write-path counterparts (`JDBCValueSetter`) are already in `JdbcUtils`.
  - Add an internal `resultSetToSparkInternalRows` method which takes a `ResultSet` and schema and returns an `Iterator[InternalRow]`. This effectively extracts the main loop of `JDBCRDD` into its own method.
  - Add a public `resultSetToRows` method to `JdbcUtils`, which wraps the minimal machinery around `resultSetToSparkInternalRows` in order to allow it to be called from outside of a Spark job.
- Make `JdbcDialect.get` into a `DeveloperApi` (`JdbcDialect` itself is already a `DeveloperApi`).

Put together, these changes enable the following testing pattern:

```scala
val jdbResultSet: ResultSet = conn.prepareStatement(query).executeQuery()
val resultSchema: StructType = JdbcUtils.getSchema(jdbResultSet, JdbcDialects.get("jdbc:postgresql"))
val jdbcRows: Seq[Row] = JdbcUtils.resultSetToRows(jdbResultSet, schema).toSeq
checkAnswer(sparkResult, jdbcRows) // in a test case
```

Author: Josh Rosen <joshrosen@databricks.com>

Closes #14907 from JoshRosen/modularize-jdbc-internals.

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