[SPARK-19089][SQL] Add support for nested sequences

## What changes were proposed in this pull request?

Replaced specific sequence encoders with generic sequence encoder to enable nesting of sequences.

Does not add support for nested arrays as that cannot be solved in this way.

## How was this patch tested?

```bash
build/mvn -DskipTests clean package && dev/run-tests
```

Additionally in Spark shell:

```
scala> Seq(Seq(Seq(1))).toDS.collect()
res0: Array[Seq[Seq[Int]]] = Array(List(List(1)))
```

Author: Michal Senkyr <mike.senkyr@gmail.com>

Closes #18011 from michalsenkyr/dataset-seq-nested.