[WIP/Tentative] Add $raw to LightGBM R Booster class

[Laurae2]

Fix for #296 .

Problems to fix:

• R6 is locked, therefore cannot add $raw easily (need to find the source but it needs to execute itself, so endless loop of requirements) - best would be adding to end of boosting, here? https://github.com/Microsoft/LightGBM/blob/master/R-package/R/lgb.train.R
• Find a way to add $raw without creating a dummy file. That would requires an in-memory of this function: https://github.com/Microsoft/LightGBM/blob/master/R-package/R/lgb.Booster.R#L169-L179
• Find a way to initialize a loaded LightGBM model via RDS. We lose all pointers. See description here: Plans to support export PMML model in R package? #296 (comment)

Currently need to read more exactly how the object is created each time to fix the first problem (most important).

Objective to get at the end of this PR (or of multiple PRs):

[msftclas]

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