Update the upsert api to handle multiple columns as unique index

[ozgrakkurt]

We want the api to look like this:

apiVersion: 0.1.0
meta:
  version: 0.3.1
  name: json-sql-connector
  type: sql-sink
  topic: sql-topic
  create-topic: true
  secrets:
    - name: DATABASE_URL
sql:
  url: ${{ secrets.DATABASE_URL }}
transforms:
  - uses: infinyon/json-sql
    with:
      mapping:
        operation: "upsert"
        table: "topic_message"
        unique-columns: 
          - device_id
          - user_id
        map-columns:
          "device_id":
            json-key: "device.device_id"
            value:
              type: "int"
              default: "0"
              required: true
          "record":
            json-key: "$"
            value:
              type: "jsonb"
              required: true

@galibey maybe we can find a better name for unique-columns? since the combination of them should be unique and they don't need to be unique individually.

[ajhunyady]

@ozgrakkurt I like parallel keywords, and since we already have map-columns, unique-columns makes sense. It also eliminates the confusion between json-key and column name.

[ozgrakkurt]

implemented in #74 but the implementation is hacky, need to update fluvio-model-sql and sql-sink to handle it as array instead of single variable named uniq_idx and add actual tests, and update docs

[ozgrakkurt]

opened follow-up issue here: #79