Add the BackfillRequest with the minimal functionality

[s-vitaliy]

Part of #144

Scope

Implemented:

• Added the BackfillRequest model

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:

  below threshold:				coverage:	threshold:
  configuration/configuration.go		 0.0% (0/18)	70%
  telemetry/app_metrics.go			 0.0% (0/9)	70%
  tests/mocks/controller.go			34.5% (10/29)	70%

Package coverage threshold (70%) satisfied:	FAIL
  below threshold:				coverage:	threshold:
  telemetry					 0.0% (0/9)	70%
  tests/mocks					68.9% (42/61)	70%
  configuration					 0.0% (0/18)	70%

Total coverage threshold (75%) satisfied:	FAIL
Total test coverage: 63.6% (96/151)

Files with uncovered lines:
  file:							coverage:	uncovered lines:
  configuration/configuration.go			0%		17-20 22-24 27-36 39-41 43-47 52-55 60
  services/controllers/stream_class/stream_class.go	85.7%		40-42 47-49 67-70 90-93 103-106 141-144
  telemetry/app_metrics.go				0%		18-22 24 53-56 60-63 65
  tests/mocks/controller.go				34.5%		47-52 55-58 61-67 70-73 90-95 98-101