fix: re-allow empty pk.fields with pk.mode set to record_key

[jclarysse]

Recently added validations (see PR #335) are preventing the support of schema evolution use-cases, e.g.

Documentation:

If 'pk.mode' is 'record_key' and 'pk.fields' is empty,
then all fields from the key struct will be used.

Configuration:

"pk.mode": "record_key",
"auto.create": "true",
"auto.evolve": "true",

Validation rule:

Primary key fields must be set when pkMode is 'record_key'.

The purpose of this PR is to re-enable this feature for record_key mode.

[jclarysse]

Create connector:

curl -X POST http://localhost:8083/connectors \
   -H "Content-Type: application/json" \
   -H "Accept: application/json" \
   -d \
  '{
    "name": "julien-jdbc-sink",
    "config": {
        "connector.class": "io.aiven.connect.jdbc.JdbcSinkConnector",
        "connection.url": "jdbc:postgresql://localhost:5432/defaultdb",
        "connection.user": "postgres",
        "connection.password": "postgres",
        "topics": "test-topic-avro",
        "key.converter": "io.confluent.connect.avro.AvroConverter",
        "key.converter.schema.registry.url": "http://localhost:8081",
        "value.converter": "io.confluent.connect.avro.AvroConverter",
        "value.converter.schema.registry.url": "http://localhost:8081",
        "pk.mode": "record_key",
        "auto.create": "true",
        "auto.evolve": "true"
    }
  }'

Produce record with Avro key via Karapace:

curl -H "Content-Type: application/vnd.kafka.avro.v2+json" -X POST -d \
  '{
      "key_schema": "{
        \"namespace\": \"example.avro\", \"type\": \"record\", \"name\": \"simplekey\",
        \"fields\": [{\"name\": \"id\", \"type\": \"int\"},{\"name\": \"code\", \"type\": \"int\"}]
      }",
      "value_schema": "{
        \"namespace\": \"example.avro\", \"type\": \"record\", \"name\": \"simplevalue\",
        \"fields\": [{\"name\": \"name\", \"type\": \"string\"}]
      }",
      "records": [{"key": {"id": 0, "code": 10}, "value": {"name": "julien"}}]
  }' https://localhost:8082/topics/test-topic-avro

Verify that the table is created with the 2 key fields as PK:

=> \d test-topic-avro
          Table "public.test-topic-avro"
 Column |  Type   | Collation | Nullable | Default 
--------+---------+-----------+----------+---------
 name   | text    |           | not null | 
 code   | integer |           | not null | 
 id     | integer |           | not null | 
Indexes:
    "test-topic-avro_pkey" PRIMARY KEY, btree (code, id)