Add support for BOM ordering

This commit adds an `order` attribute on `BillOfMaterials` that allows to
order BOMs in the generated project. Lowest value have higher priority.

When the project is generatede with a custom parent, the Spring Boot
dependencies BOM itself has an order of a 100. Any BOM that is added with
an order lower than 100 has higher priority, i.e. could potentially
override dependencies provided by the Spring Boot dependency mechanism.

This mechanism does not work reliably when using the
`spring-boot-starter-parent` so it should be used with care. However, it
can be useful when overriding dependencies that Spring Boot does not
manage itself.

Closes gh-343

initializr-web/src/test/resources/metadata/config/test-default.json

@@ -86,6 +86,7 @@
       "my-api-bom": {
         "groupId": "org.acme",
         "artifactId": "my-api-bom",
+        "order": 2147483647,
         "repositories": [],
         "additionalBoms": [
           "my-api-dependencies-bom"
@@ -117,7 +118,8 @@
         "mappings": [],
         "artifactId": "my-api-dependencies-bom",
         "additionalBoms": [],
-        "version": "1.0.0.RELEASE"
+        "version": "1.0.0.RELEASE",
+        "order": 2147483647
       }
     },
     "springBootMetadataUrl": "https://spring.io/project_metadata/spring-boot"