How to add an additional group legend on an existing corrplot?

Question

I am trying to work out the best way of approaching this visualisation problem.

I have a correlation plot that has been constructed using the following code:

library(dplyr)
library(tidyr)
library(corrplot)
library(RColorBrewer)

data <- mtcars

# Data preparation and correlation matrix calculation
df_cor <- na.omit(data) %>% 
  mutate(across(where(is.character), as.factor)) %>% 
  mutate(across(where(is.factor), as.numeric))

# Calculate the correlation matrix
cor_matrix <- cor(df_cor, method = c("spearman"))
cor_matrix 

# Calculate the p values 
res1 <- cor.mtest(df_cor, conf.level = 0.95)
p_values <- as.data.frame(res1) %>%
  rownames_to_column("rowname") %>%
  gather("var", "p", -rowname)

# Remove insignificant correlations
cor_with_p_values <- cor_matrix %>%
  as.data.frame() %>%
  rownames_to_column("rowname") %>%
  gather("var", "value", -rowname) %>%
  left_join(p_values, by = c("rowname", "var")) %>%
  mutate(value = ifelse(p > 0.05, NA, value)) %>%
  select(-p) %>%
  spread(var, value)

cor_with_p_values_only <- cor_matrix %>%
  as.data.frame() %>%
  rownames_to_column("rowname") %>%
  gather("var", "value", -rowname) %>%
  left_join(p_values, by = c("rowname", "var")) %>%
  mutate(value = ifelse(p > 0.05, NA, value)) %>%
  select(-value) %>%
  spread(var, p)

# correlation values 
filtered_cor_matrix <- cor_with_p_values %>%
  column_to_rownames("rowname") %>%
  as.matrix()

# P values 
filtered_cor_matrix_p <- cor_with_p_values_only %>%
  column_to_rownames("rowname") %>%
  as.matrix()

#ordered_cor_matrix <- filtered_cor_matrix[order_vector[order_vector %in% rownames(filtered_cor_matrix)], 
#                                          order_vector[order_vector%in% colnames(filtered_cor_matrix)]]

#ordered_cor_matrix_p <- filtered_cor_matrix_p[order_vector[order_vector %in% rownames(filtered_cor_matrix_p)], 
#                                              order_vector[order_vector%in% colnames(filtered_cor_matrix_p)]]

# Plot the correlation matrix
corrplot(filtered_cor_matrix  , is.corr = FALSE, tl.col = "black", na.label = " ",
         col = rev(brewer.pal(n = 11, name = "RdBu")),
         order = "original",
         insig = "label_sig",
         p.mat = filtered_cor_matrix_p    ,
         sig.level = c(.001, .01, .05),
         pch.cex = 0.8,
         pch.col = "black",
         method = "color", 
         type = "upper",
         diag = FALSE)

However, what I need to do is add an additional legend to the x axis to visually help the reader identify what category certain variables fall into.

Similar to adding coloured squares in this diagram:

Any guidance would be much appreciated!!!