Multiple collections support in the destination

[hariso]

Description

Closes ConduitIO/conduit#1474.

In the absence of the metadata in the Pg source connector, I've used a processor to insert the metadata:

---
version: 2.2
pipelines:
  - id: pipeline1
    status: running
    name: pipeline1
    description: 'Generator to PostgreSQL'
    connectors:
      - id: generator-source
        type: source
        plugin: "builtin:generator"
        name: generator-source
        settings:
          recordCount: "10"
          # readTime: "1s"
          format.options: "id:int,name:string"
          format.type: "structured"

      - id: pg-destination
        type: destination
        plugin: "standalone:postgres"
        name: pg-destination
        settings:
          url: "postgresql://meroxauser:meroxapass@localhost/meroxadb?sslmode=disable"
          # table: "employees"
    
    processors:
      - id: "format-lines"
        plugin: "custom.javascript"
        settings:
          script: |
            function getRandomInt(max) {
              return Math.floor(Math.random() * max);
            }

            function getRandomBoolean() {
              return Math.random() < 0.5;

            }

            function process(rec) {
              rec.Key = RawData(JSON.stringify({"id": getRandomInt(1000)}));
              if (getRandomBoolean()) {
                rec.Metadata["opencdc.collection"] = "contractors"
              } else {
                rec.Metadata["opencdc.collection"] = "employees"
              }

              return rec;
            }

Tested with a static table name and also the default Go template.

I've renamed the GitHub job build to test because it more accurately describes what it does. build is a required PR check, but I'll handle the rename once the PR has been approved.

Quick checks:

• I have followed the Code Guidelines.
• There is no other pull request for the same update/change.
• I have written unit tests.
• I have made sure that the PR is of reasonable size and can be easily reviewed.

[raulb]

LGTM! Once this ships I'll rebase #129.