Add iterator support for group collaborations

[sujaygarlanka]

Goal of this PR is to add iterator support for file, folder, and group collaborations.

• Already there for file collaborations
• Folder collaborations don't support pagination, so iterator support can't be added
• Added for group collaborations

[coveralls]

Pull Request Test Coverage Report for Build 2244

• 15 of 18 (83.33%) changed or added relevant lines in 2 files are covered.
• 6 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.01%) to 65.948%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/main/java/com/box/sdk/BoxGroup.java	7	8	87.5%
src/main/java/com/box/sdk/BoxCollaborationIterator.java	8	10	80.0%

Files with Coverage Reduction	New Missed Lines	%
src/main/java/com/box/sdk/EventStream.java	1	87.63%
src/main/java/com/box/sdk/BoxJSONResponse.java	2	93.33%
src/main/java/com/box/sdk/RealtimeServerConnection.java	3	80.0%

Totals
Change from base Build 2228:	-0.01%
Covered Lines:	5601
Relevant Lines:	8493

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💛 - Coveralls