Annotations improvements

[LesGrob]

There are to small improvements:

1. Ability to store custom user data at any annotation. Added Map<String?, Object?>? userInfo field.
2. Ability to get all annotations on current manager.

These two features make it possible to manage annotations and create scale clustering logic, for example.

[laeeqFaiz]

Any update on this?

[brandonwatkins]

Really need the ability to get all annotations :(

[akhiljohnm]

@LesGrob Just curious, does this PR include a fix for issue #902? Would be happy to collaborate on this to do so.

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