Use Gradle build scans

This commit configures Gradle build scans for the Spring Framework
build. Scans aren't performed for every build but can be enabled
with the following:

```
./gradlew build --scan
```

Issue: SPR-15885

build.gradle

@@ -15,9 +15,15 @@ buildscript {
 }
 
 plugins {
+	id "com.gradle.build-scan" version "1.8"
 	id "org.sonarqube" version "2.5"
 }
 
+buildScan {
+	licenseAgreementUrl = 'https://gradle.com/terms-of-service'
+	licenseAgree = 'yes'
+}
+
 ext {
 	linkHomepage = 'https://projects.spring.io/spring-framework'
 	linkCi = 'https://build.spring.io/browse/SPR'