Support sending multipart/form-data in RestProxy

[jianghaolu]

• Add a @PartParam and parse it in SwaggerMethodParser
• Send multipart/form-data in RestProxy

[alzimmermsft]

@jianghaolu do you have any specific service scenarios this could be tested against to verify correctness?

[jianghaolu]

Most cognitive services accept multipart/form-data as input, e.g.: https://github.com/Azure/azure-rest-api-specs/blob/2e7ca21a5ad6f749e5bd453580f1c31c1d99b4e6/specification/cognitiveservices/data-plane/ComputerVision/stable/v1.0/ComputerVision.json#L563.

[alzimmermsft]

The implementation of multipart/form-data should follow the specification laid out in RFC 7578. Based on initial review of the specification the following is known:

Option Name	Required	Default (if not required)	Explanation
Name	Yes	N/A	Name of the form field, configures name in the part Content-Disposition header
Filename	No	None	Name of a file included in the form, configures filename in the part Content-Disposition header
ContentType	No	text/plain	The Content-Type of the part, if the part is a file this will default to application/octet-stream

Depending on the usage of multipart/form-data new wrapper types may need to be created as all values above can be determined at runtime.