Add REST API example with plumber (#194)

examples/plumber/Dockerfile

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+FROM rstudio/plumber
+
+WORKDIR /app
+
+COPY . /app
+
+EXPOSE 80
+
+ENTRYPOINT ["Rscript", "main.R"]

examples/plumber/main.R

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+library(plumber)
+pr <- plumb("rest_controller.R")
+pr$run(port = 80, host = "0.0.0.0")

examples/plumber/rest_controller.R

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+#* @apiTitle Plumber Example API
+#* @apiDescription This API allows users to interact with a linear regression model predicting iris petal length from petal width.
+#* It provides endpoints for health checks, predictions, and visualizations of the model fit.
+
+# Prepare the model
+dataset <- iris
+model <- lm(Petal.Length ~ Petal.Width, data = dataset)
+
+#* Health Check - Returns the API status and the current server time
+#* @get /health-check
+function() {
+  list(
+    status = "The API is running",
+    time = Sys.time()
+  )
+}
+
+#* Predict petal length - Returns a predicted petal length for a given petal width using a linear regression model
+#* @param petal_width Numeric: Width of the petal for which length is to be predicted
+#* @post /predict_petal_length
+function(petal_width) {
+  input_data <- data.frame(Petal.Width = as.numeric(petal_width))
+  prediction <- predict(model, input_data)
+  list(petal_width = petal_width, predicted_petal_length = prediction)
+}
+
+#* Plot regression line and data points - Displays a scatter plot of the data and the regression line, illustrating the model fit
+#* @serializer png
+#* @get /plot_regression
+function() {
+  plot(dataset$Petal.Width, dataset$Petal.Length,
+    xlab = "Petal Width", ylab = "Petal Length",
+    main = "Linear Model Fit: Petal Length vs Petal Width"
+  )
+  abline(model, col = "red") # Adds the regression line in red
+}