Support XML Type on Postgres

#### Describe the feature

Postgres supports the storage of valid xml documents with the XMLType. https://www.postgresql.org/docs/9.3/datatype-xml.html
I think it would make sense to Map it to a Java String

#### Use cases

From the link above:

> The xml data type can be used to store XML data. Its advantage over storing XML data in a text field is that it checks the input values for well-formedness, and there are support functions to perform type-safe operations on it

#### Contribution

I would like to implement this but would probably need a few pointers.

I already looked at these classes (DataType.java, DataTypeCodec.java, PreparedStatementTestBase.java, RowImpl.java) and added a class to wrap the xml data which i think might be a bit redundant in the end as we could just use the String class i think. Anyway a quick talk about how a satisfactory solution could look like and what you guys think would be needed to get this going would be appreciated.

Kind regards

Inizia esaminando DataType.java, DataTypeCodec.java, PreparedStatementTestBase.java e RowImpl.java, quindi confronta il comportamento del tipo XML di PostgreSQL descritto nella documentazione collegata. Il lavoro è completo quando i valori XML validi di PostgreSQL sono supportati e mappati a Java String e i test pertinenti per prepared statement o righe coprono questo comportamento.

Scritto dal modello di indicizzazione a partire dal testo della issue.