Implement unstructured stream definition accessor

[s-vitaliy]

Part of #144

Scope

Implemented:

• Struct unstructuredWrapper which implements the StreamDefinition interface and used to access the stream definitions provided by the plugins with unstructured Kubernetes API in a safe way.

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%
  services/controllers/stream/basic_job_configurator.go		 0.0% (0/2)	70%
  telemetry/app_metrics.go					 0.0% (0/9)	70%

Package coverage threshold (70%) satisfied:	FAIL
  below threshold:				coverage:	threshold:
  configuration					 0.0% (0/18)	70%
  telemetry					 0.0% (0/9)	70%

Total coverage threshold (75%) satisfied:	FAIL
Total test coverage: 65.0% (119/183)

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/basic_job_configurator.go		0%		13-14 17-21
  services/controllers/stream/stream_definition.go		80%		69-71
  services/controllers/stream/unstructured_wrapper.go		75.6%		37-39 41-43 46-48 60-62 86-88 119-121 125-127 130-132 135-137 140-142 145-147 154-156 158-160 163-165 168-170 178-183 190-192 194-196 203-208
  services/controllers/stream_class/stream_class.go		79.4%		40-42 47-49 67-70 90-93 98-106 141-144
  telemetry/app_metrics.go					0%		18-22 24 53-56 60-63 65