Add helper function to set DashTable columns to Dash for R

[HammadTheOne]

This issue proposes adding a helper function set_columns to DashTable for R, similar to the df_to_list helper function, which would simplify the process of setting column names and id's when generating a data-table component.

With this change the syntax for a dashDataTable would go from before:

app$layout(
  dashDataTable(
    id = "table",
    columns = lapply(colnames(df), 
                     function(colName){
                       list(
                         id = colName,
                         name = colName
                       )
                     }),
    data = df_to_list(df)
  )
)

After:

app$layout(
  dashDataTable(
    id = "table",
    columns = set_columns(df)
    data = df_to_list(df)
  )
)