feat: Add nullability annotations to ClusterItem

[arriolac]

Adding nullability annotations to ClusterItem for better Kotlin-Java interop. Implementing ClusterItem will result in:

class MyModel : ClusterItem {
    override fun getPosition(): LatLng {
       // return here
    }

    override fun getTitle(): String? {
       // return here
    }

    override fun getSnippet(): String? {
       // return here
    }
}

[codecov]

Codecov Report

Merging #687 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #687   +/-   ##
=======================================
  Coverage   39.48%   39.48%           
=======================================
  Files          71       71           
  Lines        4047     4047           
  Branches      606      606           
=======================================
  Hits         1598     1598           
  Misses       2346     2346           
  Partials      103      103           

Impacted Files	Coverage Δ
...ndroid/clustering/view/DefaultClusterRenderer.java	0.00% <ø> (ø)

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[barbeau]

LGTM!

BTW, it makes me sad that you can't use Kotlin data class with fields like position to implement Java interface methods 😢:
https://youtrack.jetbrains.com/issue/KT-6653?_ga=2.30406975.1494223917.1585591891-1137021041.1573759593

[googlemaps-bot]

🎉 This PR is included in version 1.2.0 🎉

The release is available on GitHub release

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