Support `InputStream` as a source of `RequestBody`

**Is your feature request related to a problem? Please describe.**
Currently `RequestBody` can only be constructed from `ByteBuffer`. If a caller wants to supply data from an `InputStream` this API forces an allocation and copy of the data in the stream into an intermediate buffer.

**Describe the solution you'd like**
I would like to be able to create `RequestBody` from an `InputStream`. It seems that `mark` and `reset` functionality can be used to implement rewinding in `RewindableContent`.

**Describe alternatives you've considered**
I can use an intermediate buffer solution and it works but it's not ideal.

Start by locating the RequestBody construction API and the RewindableContent implementation, then compare how the existing ByteBuffer source is handled. Determine the required mark/reset behavior for an InputStream and verify that the new source avoids the intermediate copy while remaining rewindable.

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