add user in AD task

[ylwu-amzn]

Issue #, if available:

Description of changes:

1. Add user in AD task to support search AD task API
2. Add placeholder in detection state index mapping and inject detector index mapping in code. So we can only maintain detector index mapping in one place.

Test

1. ./gradlew build
2. ./gradlew integTest -PnumNodes=3
3. Start cluster and test both backend role filtering enabled and disabled cases

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[codecov]

Codecov Report

Merging #370 (889e3d3) into master (b93ec3f) will increase coverage by 0.10%.
The diff coverage is 64.00%.

@@             Coverage Diff              @@
##             master     #370      +/-   ##
============================================
+ Coverage     78.76%   78.87%   +0.10%     
+ Complexity     2653     2651       -2     
============================================
  Files           247      247              
  Lines         11695    11719      +24     
  Branches       1006     1009       +3     
============================================
+ Hits           9212     9243      +31     
+ Misses         2019     1998      -21     
- Partials        464      478      +14     

Flag	Coverage Δ	Complexity Δ
cli	79.27% <ø> (ø)	0.00 <ø> (ø)
plugin	78.84% <64.00%> (+0.11%)	0.00 <1.00> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ	Complexity Δ
...on/opendistroforelasticsearch/ad/model/ADTask.java	95.31% <52.63%> (-2.70%)	64.00 <1.00> (+1.00)	⬇️
...sticsearch/ad/indices/AnomalyDetectionIndices.java	51.26% <100.00%> (+0.62%)	45.00 <0.00> (ø)
...ndistroforelasticsearch/ad/task/ADTaskManager.java	72.64% <100.00%> (-0.17%)	79.00 <0.00> (-1.00)
...asticsearch/ad/cluster/ADClusterEventListener.java	92.00% <0.00%> (-2.00%)	14.00% <0.00%> (-1.00%)
...troforelasticsearch/ad/task/ADBatchTaskRunner.java	87.82% <0.00%> (-1.45%)	63.00% <0.00%> (-5.00%)
...earch/ad/transport/GetAnomalyDetectorResponse.java	75.00% <0.00%> (+5.95%)	10.00% <0.00%> (+2.00%)
...stroforelasticsearch/ad/model/DetectorProfile.java	37.96% <0.00%> (+8.33%)	19.00% <0.00%> (+2.00%)

[saratvemulapalli]

Thanks for the change!