feat(polygon, polyline): add path change event

[ghost]

Adds the MVCArray type, and allow to bind to changes in the paths of
AgmPolygon and AgmPolyline

[codecov]

Codecov Report

Merging #1643 into master will increase coverage by 2.6%.
The diff coverage is 67.52%.

@@            Coverage Diff            @@
##           master    #1643     +/-   ##
=========================================
+ Coverage   30.64%   33.25%   +2.6%     
=========================================
  Files          36       37      +1     
  Lines        1540     1636     +96     
  Branches      125      128      +3     
=========================================
+ Hits          472      544     +72     
- Misses       1066     1090     +24     
  Partials        2        2

Impacted Files	Coverage Δ
packages/core/services/google-maps-types.ts	0% <ø> (ø)	⬆️
packages/core/directives/polygon.ts	53.94% <24%> (-1.94%)	⬇️
packages/core/directives/polyline.ts	49.41% <33.33%> (-1.23%)	⬇️
...ackages/core/services/managers/polyline-manager.ts	63.63% <44.44%> (-4.94%)	⬇️
packages/core/utils/mvcarray-utils.ts	86.95% <86.95%> (ø)
packages/core/services/managers/polygon-manager.ts	76.78% <87.09%> (+7.82%)	⬆️

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[ghost]

fix #823