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ARROW-5883: [Java] Support dictionary encoding for List and Struct type

As described in http://arrow.apache.org/docs/format/Layout.html#dictionary-encoding, List type encoding should be supported.

Now ListVector getObject returns a ArrayList implementation, and its equals and hashCode are already overwritten, so it could be directly supported to be hashMap key in DictionaryEncoder. Since we won't change Dictionary data during encoding/decoding process, use mutable key seems dose't matter.
StructVector is similar to ListVector.

Author: tianchen <niki.lj@alibaba-inc.com>

Closes #4830 from tianchen92/ARROW-5883 and squashes the following commits:

ced12cc01 <tianchen> add helper method
03731547e <tianchen> support struct type
497753179 <tianchen> ARROW-5883:  Support Dictionary Encoding for List type