Added getter for Marker ID in maps package

[duncanhoggan]

Did this to align with the android SDK and also open up a way to associate meta data with a marker, such as linking to the underlying model associated to a marker.

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[duncanhoggan]

CLA has been added now.

[googlebot]

CLAs look good, thanks!

[amirh]

Thanks for the contribution!

LGTM, just add a comment.