adds a new "virtual" parameter `__request_body`
which can be used in http rules, see below
tremendously helps in producing arbitrary REST interfaces
for example:
```xml
<http_handlers>
<rule>
<methods>POST</methods>
<url>/ingest</url>
<handler>
<type>predefined_query_handler</type>
<query>INSERT INTO events(id, data) SELECT {id:UInt32}, {__request_body:String}</query>
</handler>
</rule>
</http_handlers>
```
