Give more options to DecodingClient

[ikhoon]

• A DecodingClient does nothing if an Accept-Encoding header is provided. The DecodingClient only works if an Accept-Encoding header is absent. It will send all possible accept encodings.
  

  armeria/core/src/main/java/com/linecorp/armeria/client/encoding/DecodingClient.java

  Lines 87 to 90 in e729261

  	if (req.headers().contains(HttpHeaderNames.ACCEPT_ENCODING)) {
  	// Client specified encoding, so we don't do anything automatically.
  	return unwrap().execute(ctx, req);
  	}

  
  Sometimes users may want to dynamically choose accept-encodings using an Accept-Encoding header.

• When unsupported Content-Encoding is received, a DecodingClient silently skip decoding. It would good that users decide whether to ignore it or returns failed response.
  

  armeria/core/src/main/java/com/linecorp/armeria/client/encoding/HttpDecodedResponse.java

  Lines 83 to 87 in ac398ea

  	// If the server returned an encoding we don't support (shouldn't happen since we set
  	// Accept-Encoding), decoding will be skipped which is ok.
  	if (decoderFactory != null) {
  	responseDecoder = decoderFactory.newDecoder(alloc);
  	}

We might give options by introducing DecodingClientBuilder. For example:

DecodingClient.buider()
              .respectAcceptEncoding(true) // Needs better names
              .ignoreUnsupportedEncoding(false)