Request: Add support for sending files as multipart form data

[slowriot]

The server can receive multipart form data, but I don't see a straightforward way to send multipart form data using the client. If this is possible already, apologies for the noise - please let me know how I should go about it. Otherwise, please add functionality to send multipart files with the client.

[yhirose]

@slowriot, thanks for the feedback. The client currently doesn't support a 'straightforward way' to send multipart form data.

Of course, you can construct HTTP header and body for mutipart form data by hand with consulting RFC 7578, since it's just a post message.

Here are my unit tests that show how to send multipart form data request:
https://github.com/yhirose/cpp-httplib/blob/master/test/test.cc#L786-L809
https://github.com/yhirose/cpp-httplib/blob/master/test/test.cc#L981-L1023

Also, you can see valuable information from the following stackoverflow post:
https://stackoverflow.com/questions/4238809/example-of-multipart-form-data

When I have time, I'll try to implement this. Thanks!