[tcgc] add support for a MultiPartFile type

[iscai-msft]

Currently, our code at large assumes models to be JSON objects. Since we need to support multipart/form-data, this is not the case. For example, in the following model

model JsonModel {
  prop: bytes
}

prop will default to a b64 encoded string, which is the correct behavior. However, in a model used for multipart, we would instead want to recognize the bytes as being for a file, and also add information about content-type and filename.

model MultipartInputModel {
  picture: bytes;
}

So proposal is to add a new primitive type like the following, which would be the above property picture's type

interface MultipartFormDataFile:
  kind: "multipartformdatafile";
  fileStream: SdkBytesType (with default encoding of `binary`, still has access to @encodedType);
  filename: string | undefined;
  contentType: string | undefined;

[iscai-msft]

Notes:

1. raise a diagnostic error if we have a model used both as multipart and as json.
2. Typing of a MultipartFormDataFile will be FileContent | [string, FileContent] | [string, FileContent, string]
3. We do not recursively set models as multipart form data. If a multipart model has a property that is a model, that property off model will be JSON

[weidongxu-microsoft]

Looks good. Java currently would have different class for different multipart file field (i.e. image field generates ImageFileDetails class), but that would be our internal processing and it could change in future if this gets to azure-core.

I assume 2nd point
Typing of a MultipartFormDataFile will be FileContent | [string, FileContent] | [string, FileContent, string]
Just means a combination the types of fileStream, filename, contentType?