Support different content types when deleting data via a transaction

[mihailradkov]

The RDF4J REST API specifies different syntax for deleting statements in a transaction.
See https://rdf4j.eclipse.org/documentation/rest-api/#the-delete-operation

Because of this, TransactionalRepositoryClient was made to receive only Turtle or TriG encoded data for the DELETE transaction operation.

Planned steps:

• Add DeleteStatementsPayload class
• Add delete() method in RDFRepositoryClient that accepts the new DeleteStatementsPayload
• Add delete() and deleteStatement() in TransactionalRepositoryClient with custom logic that maps the data from DeleteStatementsPayload to appropriate triples according to the specification.

The goal is to make both clients with same API methods and hide the custom logic in TransactionalRepositoryClient.