Attribute converter like functionality for spring-cloud-gcp-data-datastore

**Is your feature request related to a problem? Please describe.**
I'm unable to do custom conversions of specific fields between the pojo and db state, because this widely available feature is missing from spring-cloud-gcp-data-datastore, 2.0.2

**Describe the solution you'd like**
https://www.baeldung.com/jpa-attribute-converters#using-the-converter for spring-cloud-gcp-data-datastore :)

**Describe alternatives you've considered**
Without spring data support, I'm forced to use https://github.com/objectify/objectify/blob/master/src/main/java/com/googlecode/objectify/annotation/Translate.java.

**Additional context**
The goal isn't to have a global converter that converts between type A and type B (I think that is already available), but to have an annotation that can control this at the field level OR a global converter that receives the `Field` object it's converting - so for example it can check for any kind of custom annotation being present. (For example creating a `@Sensitive` annotation that triggers an encryption during the conversion. https://aws.amazon.com/blogs/developer/client-side-encryption-for-amazon-dynamodb/ does something like this with `@DoNotEncrypt`).