Support multiple components in `rl_assessment_extract()`

[willgearty]

Right now you need to do something like this to get multiple components from the same list of assessments:

df <- merge(
  rl_assessment_extract(lst, "taxon", "df", flatten = TRUE),
  rl_assessment_extract(lst, "red_list_category__code", "df", flatten = TRUE)
)

However, it would be much easier to be able to do the following:

rl_assessment_extract(lst, c("taxon", "red_list_category__code"), "df", flatten = TRUE)

where the merging would be done within rl_assessment_extract (with perhaps the caveat that this might make less sense when the extracted components have multiple rows for each assessment [e.g., locations, threats]).