API / Formatter / Add JSON and text responses support.

[fxprunayre]

Some formatter may provide text response and not only XML or PDF as
currently supported. Rename file with a proper extension based on file
type. Add support to formatter providing a text response. In such case
the XSLT formatter has to return one element like the following with the
text to be returned:

  <xsl:template match="/">
    <textResponse>
    {
    "@context": "http://schema.org",
    "@type": "Dataset",
    "@id": "https://doi.org/10.5438/4K3M-NYVG"
    ...
    }
    </textResponse>

The text response can be CSV or JSON, ...

The content type can still be controlled using accept header or the
output URL parameter.

eg.
http://localhost:8080/geonetwork/srv/api/records/ff8d8cd6-c753-4581-99a3-af23fe4c996b/formatters/jsonld?output=json

[josegar74]

Seem to test this PR is required also the changes in #3212, otherwise the Formatter API is returning a not very nice error if a formatter requested doesn't exists. The error seem not really related to these changes, but more related to the formatters API. Not sure if part of this PR, but something to improve:

http://localhost:8080/geonetwork/srv/api/records/0ad06b86-5d67-4c4c-8503-312de6cb6c8a/formatters/jsonld?output=json

javax.servlet.ServletException: org.springframework.web.util.NestedServletException: Request processing failed; nested exception is BadParameterEx : Format bundle jsonld does not exist is not a valid value for: xsl
	at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:146)
	at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:132)
	at org.eclipse.jetty.server.Server.handle(Server.java:503)
	at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:364)
	at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:260)
	at org.eclipse.jetty.io.AbstractConnection$ReadCallback.succeeded(AbstractConnection.java:305)

[fxprunayre]

Not sure if part of this PR, but something to improve:

This is not related to that PR which fix the formatter filename and default content type returned in most of the case text/html. At some point, we should maybe even declare the content type in the formatter config itself.
The error is at least almost meaningful BadParameterEx : Format bundle jsonld does not exist is not a valid value for: xsl even if we should probably improve that in the future. Maybe the formatter API should be capable of providing the list of formatter available for a schema. Maybe we could then build export menu in the UI based on the formatters available.

[fxprunayre]

but something to improve:

Created an issue for this #3241