ARROW-7113: [Rust] Add unowned buffer.
ARROW-7113: [Rust] Add unowned buffer.

Currently rust Buffer always assume that the memory passed to it is owned by itself, and frees the memory when Buffer is dropped. This is inconvenient when used in cross language environments such as jni.

Closes #5806 from liurenjie1024/arrow-7113 and squashes the following commits:

d742c3c5d <Renjie Liu> Remove from_owned
090f1b02a <Renjie Liu> fix comments
a78ae350f <Renjie Liu> Fix comments
28740f208 <Renjie Liu> Fix build break
d31058133 <Renjie Liu> Fix format
12dad4101 <Renjie Liu> Add unowned buffer

Authored-by: Renjie Liu <a144154dd2fac2f73c65000877f00109644366f2@gmail.com>
Signed-off-by: Chao Sun <f8383f764a897131a041b2260bfe2af2e49bb068@apache.org>

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