Add support for the same methods as Document convert from and to JSON.

• MongoDB\BSON\PackedArray::fromJSON
• MongoDB\BSON\PackedArray::fromPHP (already existing)
• MongoDB\BSON\PackedArray::toPHP (already existing)
• MongoDB\BSON\PackedArray::toCanonicalExtendedJSON
• MongoDB\BSON\PackedArray::toRelaxedExtendedJSON

JSON root can be an array: https://stackoverflow.com/a/3833312

For the Aggregation builder tests, I had to wrap the pipeline array into a document so that I could use `Document::fromJSON` to decode the pipeline: 

https://github.com/mongodb/mongo-php-builder/blob/d0f0565c8990cc1594d6e6fd8b7debf1f071f106/tests/Builder/PipelineTestCase.php#L22-L27