Add possibility to access raw body when executing client call

I'd like to access raw response body when my client call fails, so e.g. I can log it.

Right now if I have an endpoint: Endpoint[Unit,Unit,Nothing,Unit,Any] and do SttpClientInterpreter(SttpClientOptions.default).toRequest(endpoint, uri)(wsToPipe)(params).send(backend) I receive Response[DecodeResult[Either[E, O]]]. If this is unmapped error like 500, then we get Error(,java.lang.IllegalArgumentException: Cannot decode from: , request: GET http://localhost) which doesn't bring much value.

Usually I'd like to have the raw body in hand in such cases and log it.

I can see that in sttp there was similar issue https://github.com/softwaremill/sttp/issues/190 which was implemented with asBoth, however in tapir we don't have control over it (at least I can't see such option).

At the same time I wouldn't like to change my original endpoint to have raw String in response.

Start with SttpClientInterpreter.toRequest and the send flow described in the issue, then compare the referenced sttp asBoth approach. Define how an unmapped 500 response should expose its raw body without changing the endpoint's response type. Done means callers can access and log that body when decoding fails.