Add ability to use InputStream as source of RequestBody

[yfouquet]

Following #2424, support InputStream in RequestBody#create.

The implementation is taking feedbacks for last attempt: #1038.
As retries may be required:

• we cannot close the input stream, it remains the responsability of the caller.
• BufferedInputStream is used (instead of any InputStream) because it can be read several times.

[swankjesse]

I would prefer to not add this because the performance consequences are so unexpected. Similarly, the fact that we can’t automatically close this stream is going to be surprising to callers.

It needs to load the entire request body into memory. I’d prefer that if a caller is writing a file, they just use a separate FileInputStream on each call to writeTo().

[yschimke]

I like this and think it is a great feature I could have used myself. It is, however, likely a source of support issues for the OkHttp maintainers because of the tricky edge cases. Maybe it is better in a single purpose library to publish from your own github project? The change seems pretty clean so it doesn't need to be part of okhttp core.

[yfouquet]

Thanks for your quick feedbacks @swankjesse and @yschimke.
I'll find an alternative.