Builtin for reading request/response body

[fredmaggiowski]

Is your feature request related to a problem? Please describe

Provide a custom builtin to read the request/response body from a policy; a built-in could allow a user to avoid configuring the OAS with special flags to prevent reading the body (e.g. #273).

Note: the Built-in should have access to the original http request/reponse to be able to read the contents and refresh the buffer to make it available to be proxied

Describe the solution you'd like

It would be nice to be able to write a policy like:

my_policy {
   body := read_request_body()
}

my_policy {
   body := read_response_body()
}