feat(AgmMarker) Add drag and dragStart event support

[kommundsen]

No description provided.

[codecov]

Codecov Report

Merging #1575 into master will decrease coverage by 0.02%.
The diff coverage is 22.22%.

@@            Coverage Diff             @@
##           master    #1575      +/-   ##
==========================================
- Coverage   28.99%   28.97%   -0.03%     
==========================================
  Files          32       32              
  Lines        1452     1460       +8     
  Branches      197      173      -24     
==========================================
+ Hits          421      423       +2     
- Misses       1029     1035       +6     
  Partials        2        2

Impacted Files	Coverage Δ
packages/core/directives/marker.ts	34.31% <22.22%> (-0.8%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0e8f1ab...048a1ba. Read the comment docs.

[sebholstein]

LGTM - thanks!