Feature: Save and restore full SPARQL queries via API

[dennisvang]

Adds save/load support for the full SPARQL queries introduced in v1.19.0.

Rationale

The existing FDP implementation (<=v1.19.0) offers support for saving/loading restricted SPARQL queries by storing SearchSavedQuery objects in the relational database:

FAIRDataPoint/src/main/java/org/fairdatateam/fairdatapoint/entity/search/SearchSavedQuery.java

Line 58 in 2c893e9

private SearchQueryVariablesDTO variables;

This contains the restricted query in terms of user input, i.e. the values entered by the user:

FAIRDataPoint/src/main/java/org/fairdatapoint/api/dto/search/SearchQueryVariablesDTO.java

Lines 33 to 43 in c8d15b3

	public class SearchQueryVariablesDTO {
	@NotNull
	private String prefixes;
	@NotNull
	private String graphPattern;
	@NotNull
	private String ordering;
	}

These values are used to render the query template:

FAIRDataPoint/src/main/resources/org/fairdatapoint/service/search/queryTemplate.sparql

Lines 1 to 15 in c8d15b3

	PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
	PREFIX dct: <http://purl.org/dc/terms/>
	{{prefixes}}
	SELECT ?entity ?rdfType ?title ?description
	WHERE {
	?entity rdf:type ?rdfType .
	?entity dct:title ?title .
	OPTIONAL { ?entity dct:description ?description }
	{{graphPattern}}
	}
	ORDER BY {{ordering}}
	LIMIT 50

This PR adds the ability to save/load full SPARQL queries that are not restricted to this template.

TODO:

• ability to save raw sparql queries (in addition to the restricted sparql queries)
• ability load/run saved sparql queries

this follows up on #853

[dennisvang]

WIP: Now we can save a full sparql query, but not all HTTP methods are supported yet. I'm also not happy with the fact that we need to duplicate a lot of code. Perhaps just step back to a single changeDTO with optional fields for variables and sparqlQuery (like the actualy entity...). This may not be as clear for API users, but there would be a lot less code to maintain.