[SPARK-50599][SQL] Create the DataEncoder trait that allows for Avro and UnsafeRow encoding

### What changes were proposed in this pull request?

Currently, we use the internal byte representation to store state for stateful streaming operators in the StateStore. This PR introduces Avro serialization and deserialization capabilities in the RocksDBStateEncoder so that we can instead use Avro encoding to store state. This is currently enabled for the TransformWithState operator via SQLConf to support all functionality supported by TWS

### Why are the changes needed?

UnsafeRow is an inherently unstable format that makes no guarantees of being backwards-compatible. Therefore, if the format changes between Spark releases, this could cause StateStore corruptions. Avro is more stable, and inherently enables schema evolution.

### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

Amended and added to unit tests

### Was this patch authored or co-authored using generative AI tooling?

No

Closes #48944 from ericm-db/avro-ss.

Lead-authored-by: Eric Marnadi <[email protected]>
Co-authored-by: Eric Marnadi <[email protected]>
Signed-off-by: Jungtaek Lim <[email protected]>

Author: ericm-db