ARROW-6484: [Java] Enable create indexType for DictionaryEncoding according to dictionary value count #5321
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Related to ARROW-6484.
Currently, when create DictionaryEncoding, we need to specify indexType, and it use Int(32, true) as default if this value is null.
Actually, when dictionary valueCount is small, we should use Int(8,true)/Int(16,true) instead to reduce memory allocation.
This issue is about to provide API for creating indexType according to valueCount and apply it to avro adapter for enum type.