Adding new SMT to concatenate key with provided delimiter

## Feature request or enhancement

### Which use case/requirement will be addressed by the proposed feature?

In CDC pipelines using Debezium with **compacted topics**, key byte stability is critical.

Currently, Debezium emits structured keys (`Struct`) reflecting the database primary key. When using Avro or JSON converters, the final serialized key bytes may change due to:

* Schema evolution
* Field reordering
* Converter behavior

Kafka compares **raw key bytes**, not logical equality. If serialized bytes change:

* Compaction may fail to remove older records
* Tombstones may not match previous records
* Downstream upsert consumers may produce duplicates
* Partition routing may become unstable

We experienced this in production when using Avro keys with compacted topics.

---

## Proposed enhancement

Provide a Debezium SMT that:

* Concatenates selected key fields (typically PK fields)
* Produces a deterministic `String` key
* Uses a configurable delimiter
* Ensures stable field order

Example:

Database PK:

```sql
PRIMARY KEY (currency, isin, stock_exchange)
```

Structured key → transformed key:

```
CHF|CH1234567890|SIX
```

This ensures:

* Byte-level determinism
* Converter independence
* Stable partitioning
* Safe compaction behavior

---

## Implementation ideas (optional)

Config example:

```properties
transforms=keyConcat
transforms.keyConcat.type=io.debezium.transforms.KeyConcatenation
transforms.keyConcat.fields=currency,isin,stock_exchange
transforms.keyConcat.delimiter=|
```

Options:

* `fields` (default: all key fields in schema order)
* `delimiter` (default: `|`)
* `null.handling` (fail / empty / literal)

Output key schema: `Schema.STRING_SCHEMA`.

We have implemented internally but would be happy to contribute to dbz open source if people are interested.