[R-package] introduce Dataset methods set_field() and get_field() (#4571)

* [R-package] introduce Dataset set_field() and get_field()

* fix incorrect fields

* update pkgdown

* fix example

* fix another example

* Apply suggestions from code review
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

* update docs
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

R-package/tests/testthat/test_lgb.plot.interpretation.R

@@ -11,10 +11,10 @@ test_that("lgb.plot.interepretation works as expected for binary classification"
     data(agaricus.train, package = "lightgbm")
     train <- agaricus.train
     dtrain <- lgb.Dataset(train$data, label = train$label)
-    setinfo(
+    set_field(
         dataset = dtrain
-        , "init_score"
-        , rep(
+        , field_name = "init_score"
+        , data = rep(
             .logit(mean(train$label))
             , length(train$label)
         )