Implement web-hook to send mapped payload to external custom endpoints

[ck-c8y]

This issues sketches the required changes to address a web-hook for to send mapped payload to external custom endpoints.
This was requested in #303.

We plan to create a new connector WebHook, e.g. dynamic.mapping.connector.http.WebHook and leave the existing Http Connector unchanged. The web-hook is only for outbound and has custom properties, whereas the already existing http Connector is a singleton and only has one property.

The custom properties of the web-hook could be:

1. Base URL incl. host, port, context path, e.g. https://myServer.com:8001/webHook
2. Different authorization mechanisms: basic, bearer token, ...
3. content-type (some specialization of application/json)

Payload will be the mapped payload.

Possible properties in the web-hook could be specified in the code as follows:

        Map<String, ConnectorProperty> configProps = new HashMap<>();
	configProps.put("url",
			new ConnectorProperty(true, 0, ConnectorPropertyType.STRING_PROPERTY, false, false, null, null));
	configProps.put("user",
			new ConnectorProperty(false, 1, ConnectorPropertyType.STRING_PROPERTY, false, false, null, null));
	configProps.put("password",
			new ConnectorProperty(false, 2, ConnectorPropertyType.SENSITIVE_STRING_PROPERTY, false, false, null,
				null));
        String name = "Web Hook";
        String description = "Generic Web Hook to send custom payload in the body......\n" ;
        connectorType = ConnectorType.WEB_HOOK;
        connectorSpecification = new ConnectorSpecification(name, description, connectorType, configProps, false);

Tasks backend

1. Implement dynamic.mapping.connector.http.WebHook
2. Change method public boolean supportsOutbound() in dynamic.mapping.connector.core.client.AConnectorClient to public List<Direction> supportedDirections()
3. Add WEB_HOOK to dynamic.mapping.connector.core.client.ConnectorType
4. Registe web-hook in registerDefaultConnectors()