[Feature/extensions] Get namedXContentRegistry from ExtensionsRunner

[dbwiddis]

Note: Requires SDK #244 to be merged first.

Description

Simplifies AnomalyDetectorExtension:

• Inherits from BaseExtension to simplify extension settings and remove unneeded overrides
• Passes its NamedXContent to the ExtensionsRunner during init, to be merged with core NamedXContent
• Obtains the merged namedXContentRegistry from the Extensions Runner and passes it to the REST handler

Issues Resolved

SDK #208

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[owaiskazi19]

@dbwiddis looks like you have to rebase with feature/extensions to get the successful build?

[codecov-commenter]

Codecov Report

Merging #725 (19c08f1) into feature/extensions (11d094a) will decrease coverage by 0.46%.
The diff coverage is 16.41%.

@@                   Coverage Diff                    @@
##             feature/extensions     #725      +/-   ##
========================================================
- Coverage                 53.81%   53.35%   -0.47%     
+ Complexity                 2679     2658      -21     
========================================================
  Files                       288      290       +2     
  Lines                     16004    16056      +52     
  Branches                   1703     1686      -17     
========================================================
- Hits                       8612     8566      -46     
- Misses                     6767     6872     +105     
+ Partials                    625      618       -7     

Flag	Coverage Δ
plugin	53.35% <16.41%> (-0.47%)	⬇️

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

Impacted Files	Coverage Δ
...va/org/opensearch/ad/AnomalyDetectorExtension.java	0.00% <0.00%> (ø)
...main/java/org/opensearch/ad/auth/UserIdentity.java	54.78% <ø> (ø)
.../java/org/opensearch/ad/cluster/ADVersionUtil.java	80.00% <ø> (+5.00%)	⬆️
...in/java/org/opensearch/ad/model/ADTaskProfile.java	64.97% <0.00%> (ø)
.../java/org/opensearch/ad/model/AnomalyDetector.java	90.00% <ø> (ø)
...g/opensearch/ad/rest/RestCreateDetectorAction.java	0.00% <0.00%> (ø)
.../org/opensearch/ad/rest/RestGetDetectorAction.java	0.00% <0.00%> (ø)
...ensearch/ad/transport/ADCancelTaskNodeRequest.java	34.78% <ø> (ø)
...rg/opensearch/ad/transport/ADStatsNodeRequest.java	100.00% <ø> (ø)
...nsearch/ad/transport/ADTaskProfileNodeRequest.java	0.00% <ø> (ø)
... and 21 more