I want to add a `predefined_query_handler` to serve a single JSON from my query.
For example

```xml
<rule>
      <url>/my/url</url>
      <methods>GET</methods>
      <handler>
        <type>predefined_query_handler</type>
        <query>select '{"my": "json"}' format JSONAsString</query>
      </handler>
    </rule>
```

I can use `RawBLOB` but it sets up Content-Type 'text/plain' instead of 'application/json'.
I think JSONAsString should also work with output.

#include <Server/HTTPHandler.h>
#include <Access/Authentication.h>
#include <Access/Credentials.h>
#include <Access/ExternalAuthenticators.h>
#include <Compression/CompressedReadBuffer.h>
#include <Compression/CompressedWriteBuffer.h>
#include <Core/ExternalTable.h>
#include <Disks/StoragePolicy.h>
#include <IO/CascadeWriteBuffer.h>
#include <IO/ConcatReadBuffer.h>
#include <IO/MemoryReadWriteBuffer.h>
#include <IO/ReadBufferFromString.h>
#include <IO/WriteBufferFromTemporaryFile.h>
#include <IO/WriteHelpers.h>
#include <IO/copyData.h>
#include <Interpreters/Context.h>
#include <Interpreters/QueryParameterVisitor.h>
#include <Interpreters/executeQuery.h>
#include <Interpreters/Session.h>
#include <Server/HTTPHandlerFactory.h>
#include <Server/HTTPHandlerRequestFilter.h>
#include <Server/IServer.h>
#include <Common/logger_useful.h>
#include <Common/SettingsChanges.h>