[core] allow to specify a test state sequence upon DCS requests

When ECS2DCS2ECS is used as the DCS server, one can specify expected reply to requests by adding "STATE_SEQUENCE" to request extra parameters with a key formatted as below:
```
MILLIS_DELAY:DETECTOR_STATE[,MILLIS_DELAY:DETECTOR_STATE]*
```
This commit exposes a possibility to set these sequences via configuration parameters.
Essentially, this allows us to have some configurations on staging which use DCS with pre-defined state sequences.

Author: knopers8

core/integration/dcs/plugin.go

@@ -640,6 +640,13 @@ func (p *Plugin) CallStack(data interface{}) (stack map[string]interface{}) {
 			detectorArgMap = addEnabledLinks(detectorArgMap, varStack, ecsDet,
 				log.WithField("detector", ecsDet))
 
+			// a state sequence that can be defined in order to get an expected answer, useful for testing.
+			// format: MILLIS_DELAY:DETECTOR_STATE[,MILLIS_DELAY:DETECTOR_STATE]*
+			pfrStateSequence, ok := varStack["dcs_pfr_state_sequence"]
+			if ok {
+				detectorArgMap["STATE_SEQUENCE"] = pfrStateSequence
+			}
+
 			in.Detectors[i] = &dcspb.DetectorOperationRequest{
 				Detector:        dcsDet,
 				ExtraParameters: detectorArgMap,
@@ -1255,6 +1262,13 @@ func (p *Plugin) CallStack(data interface{}) (stack map[string]interface{}) {
 			detectorArgMap = addEnabledLinks(detectorArgMap, varStack, ecsDet,
 				log.WithField("detector", ecsDet))
 
+			// a state sequence that can be defined in order to get an expected answer, useful for testing.
+			// format: MILLIS_DELAY:DETECTOR_STATE[,MILLIS_DELAY:DETECTOR_STATE]*
+			sorStateSequence, ok := varStack["dcs_sor_state_sequence"]
+			if ok {
+				detectorArgMap["STATE_SEQUENCE"] = sorStateSequence
+			}
+
 			in.Detectors[i] = &dcspb.DetectorOperationRequest{
 				Detector:        dcsDet,
 				ExtraParameters: detectorArgMap,
@@ -1740,6 +1754,12 @@ func (p *Plugin) CallStack(data interface{}) (stack map[string]interface{}) {
 
 				return
 			}
+			// a state sequence that can be defined in order to get an expected answer, useful for testing.
+			// format: MILLIS_DELAY:DETECTOR_STATE[,MILLIS_DELAY:DETECTOR_STATE]*
+			eorStateSequence, ok := varStack["dcs_eor_state_sequence"]
+			if ok {
+				detectorArgMap["STATE_SEQUENCE"] = eorStateSequence
+			}
 
 			in.Detectors[i] = &dcspb.DetectorOperationRequest{
 				Detector:        dcsDet,