arrow

ARROW-3757: [R] R bindings for Flight RPC client

This provides some basic methods for interacting with a Flight RPC server via `pyarrow` and `reticulate`.

Closes #7875 from nealrichardson/r-flight

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>