feat(AgmMap): Add tilesloaded event to AgmMap

[CorneeSean]

This pull request exposes 'tilesloaded' event on AgmMap component. GoogleMaps API reference here.
Closes #1701

[codecov]

Codecov Report

Merging #1706 into master will decrease coverage by 0.13%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1706      +/-   ##
==========================================
- Coverage   39.96%   39.83%   -0.14%     
==========================================
  Files          41       41              
  Lines        1799     1805       +6     
  Branches      154      165      +11     
==========================================
  Hits          719      719              
- Misses       1079     1083       +4     
- Partials        1        3       +2

Impacted Files	Coverage Δ
packages/core/directives/map.ts	0% <0%> (ø)	⬆️
...ackages/core/services/managers/polyline-manager.ts	70.14% <0%> (ø)	⬆️

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[CorneeSean]

From what I see, there are no test files related to AgmMap component so far, hence no tests added by me. This decreases coverage though.