set user as nested type to support exists query

[ylwu-amzn]

Issue #, if available:

Description of changes:
We need to set the user type as nested, then we can use exists query to check if user and user.backend_roles exists

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[codecov]

Codecov Report

Merging #291 into master will decrease coverage by 0.08%.
The diff coverage is 77.77%.

@@             Coverage Diff              @@
##             master     #291      +/-   ##
============================================
- Coverage     71.39%   71.31%   -0.09%     
+ Complexity     1871     1870       -1     
============================================
  Files           195      195              
  Lines          9020     9018       -2     
  Branches        764      764              
============================================
- Hits           6440     6431       -9     
- Misses         2215     2221       +6     
- Partials        365      366       +1     

Flag	Coverage Δ	Complexity Δ
#cli	79.27% <ø> (ø)	0.00 <ø> (ø)

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

Impacted Files	Coverage Δ	Complexity Δ
...stroforelasticsearch/ad/AnomalyDetectorPlugin.java	93.44% <ø> (ø)	13.00 <0.00> (ø)
...est/handler/IndexAnomalyDetectorActionHandler.java	48.77% <ø> (ø)	32.00 <0.00> (ø)
...ticsearch/ad/settings/AnomalyDetectorSettings.java	100.00% <ø> (ø)	1.00 <0.00> (ø)
...ransport/SearchAnomalyDetectorTransportAction.java	35.71% <0.00%> (ø)	4.00 <0.00> (ø)
...rch/ad/transport/IndexAnomalyDetectorResponse.java	75.00% <85.71%> (ø)	4.00 <1.00> (ø)
...d/transport/GetAnomalyDetectorTransportAction.java	44.32% <100.00%> (-2.55%)	9.00 <0.00> (ø)
...asticsearch/ad/cluster/ADClusterEventListener.java	88.00% <0.00%> (-4.00%)	13.00% <0.00%> (-1.00%)
...ransport/DeleteAnomalyDetectorTransportAction.java	57.95% <0.00%> (-2.28%)	16.00% <0.00%> (ø%)
... and 2 more

[saratvemulapalli]

Looks good to me!