R investigate and fix suspicious field assignments

[tomasfryda]

When trying to build a conda R package the following warnings were emitted. This might be a blocker for future CRAN release so it should be investigated and dealt with. I can reproduce it when running conda build with meta.yaml mentioned on https://github.com/h2oai/h2o-3/issues/16144.

code for methods in class "H2OConnectionMutableState" was not checked for suspicious field assignments (recommended package 'codetools' not available?)
code for methods in class "model_cache" was not checked for suspicious field assignments (recommended package 'codetools' not available?)
code for methods in class "models_info" was not checked for suspicious field assignments (recommended package 'codetools' not available?)