[R-package] reduce cost of repeated parameter alias checks

[jameslamb]

Created in response to #5122 (comment).

For many of LightGBM's supported parameters (https://lightgbm.readthedocs.io/en/latest/Parameters.html), LightGBM respects multiple aliases. For example, num_iterations=10, nrounds=10, and num_tree=10 all result in the same behavior.

As a result of this, the R package frequently needs to deal with the possibility that some relevant configuration might be provided via any of these aliases, and sometimes via special keyword arguments in the R package's public API.

Earlier versions of {lightgbm} included a hard-coded list of string literals with a mapping from select "main" parameter names to their aliases. To ensure that the package is always in sync with changes on the C++ side, #4829 replaced that list with a call to an entrypoint in LightGBM's C++ code which returns a JSON string containing all parameters and their aliases.

As noted in #5122 (comment), each of those calls results in re-generating that JSON string and deserializing it into an R list.

Since the parameter mapping will never change within an R session, it should only be necessary to fetch it once. This PR proposes introducing an internal cache in the R package and changing .PARAMETER_ALIASES() to check that cache before going to the C++ side to get parameter aliases.

[StrikerRUS]

Very nice indeed!

[github-actions]

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