Make it easier to apply existing `Content` functions to lists of content

#463 (in flight at time of writing) modifies search_content() to return a list of Connect class objects.

Right now, the best way to call existing functions that operate on Content class objects is to use purrr::map(). This is pretty simple if there are no additional arguments passed, but if additional args were required, you'd have to use R's anonymous function syntax.

# Assume `old_content` is a list of content class objects that need to be locked.

length(old_content)
# [1] 48

# Use purrr::map to apply existing functions to the list.
old_content <- purrr::map(old_content, lock_content)

# When passing extra args
old_content <- purrr::map(old_content, \(x) lock_content(x, locked_message = "R version is too old."))

It would be nicer if you could just pass the list to the functions and have it work, like so:

old_content <- lock_content(old_content, locked_message = "R version is too old.")

This might require us modifying all the content functions.

Read issue #463's change to search_content() and inspect the existing functions that operate on Content objects; no file or test is named here. Define how list inputs and additional arguments should behave, then verify that calls such as lock_content(old_content, locked_message = "R version is too old.") match the purrr::map() result.