Add mapper.policyToTargetRefObj() helper function

[zijun726911]

Changes

• Add a helper resourceMapper.policyToTargetRefObj() so that both VpcAssociationPolicyToGateway() and TargetGroupPolicyToService could leverage it
• Mimic Refactor event handling and remove service reconciler #360, Add the vpcAssociationPolicyEventHandler.MapToGateway() method
• Make gateway_controller optionally watches the VpcAssociationPolicy

What type of PR is this? Prepare for SNVA Security Group Api support #199

What does this PR do / Why do we need it: Prepare for SNVA Security Group Api support

Testing done on this change:

• More unit test for mapper.go
• Manual test done: kubectl apply -f examples/vpc-association-policy.yaml could enter the gatewayReconciler.Reconcile() method if targetRef gateway exists
• E2E test all pass

Ran 31 of 31 Specs in 2006.710 seconds
SUCCESS! -- 31 Passed | 0 Failed | 0 Pending | 0 Skipped
--- PASS: TestIntegration (2007.08s)
PASS

Will this PR introduce any new dependencies?: No

Will this break upgrades or downgrades. Has updating a running cluster been tested?: No

Does this PR introduce any user-facing change?: No

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[coveralls]

Pull Request Test Coverage Report for Build 6240134816

• 42 of 84 (50.0%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.002%) to 38.666%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
controllers/eventhandlers/vpcAssociationPolicy.go	0	11	0.0%
controllers/eventhandlers/mapper.go	42	57	73.68%
controllers/gateway_controller.go	0	16	0.0%

Files with Coverage Reduction	New Missed Lines	%
controllers/eventhandlers/mapper.go	1	72.66%

Totals
Change from base Build 6227070936:	0.002%
Covered Lines:	3966
Relevant Lines:	10257

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💛 - Coveralls

[scottlaiaws]

LGTM!