Read json format of gbtree and gblinear

[tonylee19544]

This will add support for json format and binary format of gbtree and gblinear which have the version not younger than v1.0.0

[dmitryikh]

@tonylee19544 , great job! That's definitely what leaves needs!

I will come back yo you with review after the easter holidays.

[tonylee19544]

Hi would like to ask if there is some news for the pull request. Thanks.

[bgweber]

@dmitryikh Can you review? Super useful library and the json bindings help modernize!

[ruthlee]

Yes!! Would love to see this merged @dmitryikh

[adensur]

Have a problem with this pull request.
I have an xgboost model trained with version 1.6.0. Training code: https://gist.github.com/adensur/134b82c304cc69eed278e258e6b2a685
json read error: json: cannot unmarshal number into Go struct field Tree.learner.gradient_booster.model.trees.default_left of type bool
"default_left" field in tree nodes are ints (in current xgboost version, at least), not bools.
After fixing that, the prediction result is incorrect. I don't understand where this formula is coming from: https://github.com/dmitryikh/leaves/pull/81/files#diff-c524a0589fd250b78ba6e81dbd14998971af6ea9fd8dd2aad8c2dd8e3ecf6a4dR334
But without it prediction is correct, in my case at least