implement HttpRequestBodyStream for files

[aajtodd]

We should probably consider wrapping java.io.File into something that implements HttpRequestBodyStream (or better yet special casing it to just use the existing aws_input_stream implementation: https://github.com/awslabs/aws-c-io/blob/main/include/aws/io/stream.h#L94 and avoid going through JNI all together (for reading the file))

[jmklix]

This isn't needed anymore: awslabs/aws-sdk-kotlin#227 (comment)

We've significantly refactored the I/O model as well as implemented signing directly in Kotlin since this issue was opened. Closing as there is no signs this is actually any kind of bottleneck, we can always re-evaluate with concrete use cases/issues if that isn't the case.