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ARROW-6484: [Java] Enable create indexType for DictionaryEncoding according to dictionary value count

Related to [ARROW-6484](https://issues.apache.org/jira/browse/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.

Closes #5321 from tianchen92/ARROW-6484 and squashes the following commits:

fc1931b4b <tianchen>  Enable create indexType for DictionaryEncoding according to dictionary value count

Authored-by: tianchen <niki.lj@alibaba-inc.com>
Signed-off-by: Micah Kornfield <emkornfield@gmail.com>