Change the non-obvious execution settings layout

[s-vitaliy]

Fixes #305

Scope

As discussed, the new layout is the following:

  execution:
    streamingBackend:
      batch:
        schedule: "*/30 * * * *"
        jobTemplateRef:
          kind: StreamingJobTemplate
          name: job-template
          namespace: default

or

  execution:
    streamingBackend:
      changeTracking:
        backfillJobTemplateRef:
          kind: StreamingJobTemplate
          name: backfill-template
          namespace: default
        jobTemplateRef:
          kind: StreamingJobTemplate
          name: job-template
          namespace: backfill

NOTE the layout version will not be changed since the change affects beta streams only.

Checklist

• GitHub issue exists for this change.
• Unit tests added and they pass.
• Line Coverage is at least 80%.
• Review requested on latest commit.

[github-actions]

go-test-coverage report:

Package coverage threshold (70%) satisfied:	PASS
Total coverage threshold (75%) satisfied:	PASS
Total test coverage: 88.3% (709/803)