[Swift] Add parsing from unowned UnsafeMutableRawPointer for ByteBuffer (#5981)

[Swift] Add parsing from unowned UnsafeMutableRawPointer for ByteBuffer (#5981)

* RFC: Add ExternalStorage for ByteBuffer in Swift implementation

This PR proposed one more API for ByteBuffer such that no copy is
required to parse FlatBuffers content. This API has limited use, but for
cases that you need to read part of the flatbuffers' data to decide
whether you want to parse / copy the full buffer out, it is useful.

* Use a variable rather than protocol.

* Revert grouping changes from the other PR.

* Add unit test to read from unowned UnsafePointer.

* Manifest changed.