Allow transmitter calls to use caFilePath

`caFilePath` config option was only used by the agent. With this change,
the certificate file will also be used by the diagnose transmitter to
validate the api key and to send the diagnose results to AppSignal.

packages/nodejs/.changesets/cafilepath-works-for-diagnose-script-external-requests.md

@@ -0,0 +1,6 @@
+---
+bump: "patch"
+type: "add"
+---
+
+The `caFilePath` config option now works for diagnose script external requests.

packages/nodejs/src/diagnose.ts

@@ -253,7 +253,9 @@ export class DiagnoseTool {
           "  Error: Something went wrong while submitting the report to AppSignal."
         )
         console.error(`  Response code: ${responseData["status"]}`)
-        console.error(`  Response body:\n${responseData["body"]}`)
+        console.error(
+          `  Response body:\n${JSON.stringify(responseData["body"])}`
+        )
       })
   }
 }

packages/nodejs/src/transmitter.ts

@@ -75,10 +75,22 @@ export class Transmitter {
         "Content-Type": "application/json",
         "Content-Length": this.#data.length
       },
-      cert: fs.readFileSync(
-        path.resolve(__dirname, "../cert/cacert.pem"),
-        "utf-8"
+      ca: this.certificateFile()
+    }
+  }
+
+  private certificateFile(): string {
+    const configData = this.#config.data
+    const caFilePathFromConfig = configData["caFilePath"] || ""
+
+    try {
+      fs.accessSync(caFilePathFromConfig, fs.constants.R_OK)
+      return fs.readFileSync(caFilePathFromConfig, "utf-8").toString()
+    } catch {
+      console.warn(
+        `Provided caFilePath: '${caFilePathFromConfig}' is not readable.`
       )
+      return ""
     }
   }
 }