`span`-ification: `mojo::DataPipe[Consumer|Producer]Handle`: New API.

This CL `span`-ifies the APIs in `mojo/public/cpp/system/data_pipe.h`:

* `BeginReadData` and `BeginWriteData` return (via an out parameter)
  a `span` instead of returning a ptr+size pair.
* `ReadData` and `WriteData` consume a `span` instead of consuming
  a ptr+size pair.

This CL adds the new APIs + coverts callers under `//mojo`.

For more details see the design note here:
https://docs.google.com/document/d/1c4NKpXwpQ9MKK1SbJ4C6MvhXI8-KJZ4jq7N4VHTHJoI/edit?usp=sharing

Bug: 40284755
Change-Id: I70a7fab6e57eb3481305e82dd504de91042fa881
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5598233


Auto-Submit: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313022}