[FEATURE REQ] clientcore annotation-processor, handle BinaryData on file upload

[weidongxu-microsoft]

Is your feature request related to a problem? Please describe.
annotation-processor 1.0.0-beta.4

TypeSpec test on upload JSON as file
https://github.com/microsoft/typespec/blob/main/packages/http-specs/specs/type/file/main.tsp#L27-L36

TypeSpec doc on File https://typespec.io/docs/libraries/http/files
See "contentType" part

also https://typespec.io/docs/libraries/http/files/#creating-custom-file-models

So, I think the REST API would have content-type header as application/json in the request, and the body is JSON encoded as UTF-8 (e.g. {"key", "value"}).

Generated code be
https://github.com/microsoft/typespec/blob/main/packages/http-client-java/generator/http-client-generator-clientcore-test/src/main/java/type/file/FileClient.java#L85-L107

However, the ServiceImpl generated by annotation-processor would do another BinaryData.fromObject(), which make the payload body "{\"key\", \"value\"}".

    @Override
    public Response<Void> uploadFileJsonContentType(String endpoint, String contentType, BinaryData file, long contentLength, RequestContext requestContext) {
        // Create the HttpRequest.
        HttpRequest httpRequest = new HttpRequest().setMethod(HttpMethod.POST).setUri(endpoint + "/type/file/body/request/json-content-type");
        httpRequest.getHeaders().add(new HttpHeader(HttpHeaderName.CONTENT_LENGTH, String.valueOf(contentLength)));
        if (file != null) {
            httpRequest.getHeaders().set(HttpHeaderName.CONTENT_TYPE, contentType);
            SerializationFormat serializationFormat = CoreUtils.serializationFormatFromContentType(httpRequest.getHeaders());
            if (xmlSerializer.supportsFormat(serializationFormat)) {
                httpRequest.setBody(BinaryData.fromObject(file, xmlSerializer));
            } else {
                httpRequest.setBody(BinaryData.fromObject(file, jsonSerializer));
            }
        }
        httpRequest.setContext(requestContext);
        httpRequest.getContext().getRequestCallback().accept(httpRequest);

Describe the solution you'd like
The current behavior of annotation-processor seems wrong. If the request body is already a BinaryData, it should not process it again.

PS: code in azure-core DPG passes. There the distinction is the type. If it is byte[], the payload would be processed as a JSON string of base64 encode of the byte[]. If it is BinaryData, it is the exact payload.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
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Information Checklist
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[github-actions]

@browndav-msft @gunjansingh-msft @ibrandes @kyleknap @seanmcc-msft

[github-actions]

Thank you for your feedback. Tagging and routing to the team member best able to assist.

[weidongxu-microsoft]

Had a quick discussion with Srikanta (before send the issue).

There could be certain confusion of what this "upload JSON as file" mean in REST API level:

1. upload with content-type=octet/stream, and body as that JSON encoded as octet encoding (the "file" approach)
2. upload with content-type=application/json, and body as that JSON encoded as utf-8 (the "json" approach)

From the TypeSpec doc on File, I think it means the latter case (and the mock in the test also match this).

If we'd like to, we can have further discussion with TypeSpec dev.