ConfigurationPropertiesPlugin should allow to use manual configuration metadata only

[snicoll]

We have modules that only have manual configuration metadata. For those, we don't use META-INF/spring/additional-configuration-metadata.json as this file is used by the annotation processor to merge metadata from generated ones. And the AP won't kick-in for a project that doesn't have any code with automatic metadata.

Those projects only have a static META-INF/spring/configuration-metadata.json and do not apply the org.springframework.boot.configuration-properties plugin.

Because of the former no check is applied on the file that's still written manually.

[snicoll]

@wilkinsona and I discussed this and we believe that the plugin, or a second plugin, should be able to handle the case of a module having only manual metadata. There are several reasons to this:

• The full check is only performed on the generated metadata. Since we don't apply the plugin, checks on manual metadata only isn't performed
• On main we use the activation of the plugin to source the required metadata and generate configuration properties in the documentation. Modules that only have manual metadata aren't documented at the moment.