WIP: [ci] consolidate R package installs into a CI script

[jameslamb]

Follow-up to #6804 (comment)

Consolidates the various install.packages() calls across the project's CI into a single .ci/ script.

Notes for Reviewers

I'm intentionally not proposing updating user-facing docs like this:

LightGBM/R-package/README.md

Lines 429 to 431 in 425395d

	# install dependencies
	RDscript${R_CUSTOMIZATION} \
	-e "install.packages(c('R6', 'data.table', 'jsonlite', 'knitr', 'markdown', 'Matrix', 'RhpcBLASctl', 'testthat'), repos = 'https://cran.r-project.org', Ncpus = parallel::detectCores())"

I'd like to keep this script limited to LightGBM's CI, and changeable at any time. I don't want to make it a part of the public interface of the project or encourage anyone else to use it.