Feature/add workflow completion listener al

[ghost]

Adding a listener for when workflow gets to a COMPLETED or TERMINATED state.
This listener could be redefined in projects that use or inherit conductor library with some custom implementation as needed.

WorkflowDefinition has a flag to trigger these listeners so that we do not trigger these for every workflow.

[codecov]

Codecov Report

Merging #846 into dev will increase coverage by 0.02%.
The diff coverage is 72.72%.

@@             Coverage Diff              @@
##                dev     #846      +/-   ##
============================================
+ Coverage     58.43%   58.46%   +0.02%     
- Complexity     2212     2219       +7     
============================================
  Files           207      209       +2     
  Lines         11154    11176      +22     
  Branches       1056     1058       +2     
============================================
+ Hits           6518     6534      +16     
- Misses         4132     4137       +5     
- Partials        504      505       +1

Impacted Files	Coverage Δ	Complexity Δ
...nductor/core/execution/WorkflowExecutorModule.java	100% <100%> (ø)	2 <2> (?)
...m/netflix/conductor/bootstrap/ModulesProvider.java	58.18% <100%> (+0.77%)	4 <0> (ø)	⬇️
...onductor/common/metadata/workflow/WorkflowDef.java	78.94% <100%> (+1.16%)	36 <2> (+2)	⬆️
...om/netflix/conductor/grpc/AbstractProtoMapper.java	39.06% <100%> (+0.15%)	54 <0> (ø)	⬇️
...tor/core/execution/WorkflowStatusListenerStub.java	33.33% <33.33%> (ø)	2 <2> (?)
...lix/conductor/core/execution/WorkflowExecutor.java	70.89% <60%> (-0.1%)	108 <0> (+1)

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[coveralls]

Pull Request Test Coverage Report for Build 1993

• 17 of 22 (77.27%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 63.019%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core/src/main/java/com/netflix/conductor/core/execution/WorkflowExecutor.java	4	5	80.0%
core/src/main/java/com/netflix/conductor/core/execution/WorkflowStatusListenerStub.java	2	6	33.33%

Totals
Change from base Build 1991:	0.03%
Covered Lines:	7043
Relevant Lines:	11176

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💛 - Coveralls