Support different JsonMapper for different datasources

### Feature description

I have a use-case where it would be helpful for Micronaut to support having different JsonMappers for different datasources. Specifically, I need to use a different JSON serialization strategy for a DataType.JSON field I am persisting with Micronaut Data JDBC datasource than the default JsonMapper that I use to serialize my API's HTTP responses, so if I could define a `@Named` JsonMapper and then configure a datasource to use a specific named JsonMapper instead, that would solve my problem!

No file or test is named in the issue. Start by tracing how DataType.JSON persistence selects the JsonMapper for a Micronaut Data JDBC datasource; done means a datasource can select a named mapper independently of the default mapper used for HTTP responses, with coverage for both configurations.

Written by the indexing model from the issue text.