Fix Metadata serialization (serialize null values)

[valentinEmpy]

As per documentation in order to delete labels/annotations from a resource's metadata, one must:

To delete, include the existing key with a null value.

Explained in: https://v3-apidocs.cloudfoundry.org/version/3.80.0/index.html#Using-labels-and-annotations

The Immutables class includes AllowNulls annotation but does not have JsonInclude. This makes the default ObjectMapper which is initialized using setSerializationInclusion(NON_NULL) to IGNORE the null mappings in the label/annotation maps and does not serialize them when sending the request to the controller. This in turn, does not allow the deletion of Metadata keys at all.
Using JsonInclude annotation overriding the default serialization inclusion handles this issue.

[valentinEmpy]

There's the possibility to provide a custom ObjectMapper with a different default serialization inclusion but this would lead to side effects of other calls we do, hence there is no way for us to delete metadata labels/annotations.