Add Spring Session BOM module

With Spring Session Data Geode module being removed from the BOM, all of Spring Session's modules are now managed by this repository.

This means that the BOM itself can be moved to this repository, in order to simplify the overall project maintenance.

See gh-2195

spring-session-bom/spring-session-bom.gradle

@@ -0,0 +1,15 @@
+import io.spring.gradle.convention.SpringModulePlugin
+
+plugins {
+	id("io.spring.convention.bom")
+}
+
+dependencies {
+	constraints {
+		project.rootProject.allprojects { project ->
+			project.plugins.withType(SpringModulePlugin) {
+				api(project)
+			}
+		}
+	}
+}