Add support for so-called 'simplified' format, including 'flat JSON' format to the REST APIs.
Specify Content-Type and Accept header.
The link /json_data_template.html in src/includes/overview.apib does not seem to work on the generated webpage, it points to https://specifications.openehr.org/json_data_template.html but needs to point to (e.g.) https://specifications.openehr.org/releases/ITS-REST/latest/json_data_template.html - losing the initial / may do the trick?
Note that the JSON Data Template spec is not finished. But we can add it anyway. Does this CR add a HTTP header tag to REST calls that want to use this format or is that already done?
This issue will require further works on upcoming releases, but in meanwhile it will be closed the way it is (stating 3 simplified formats)