Consolidate Buffer -> byte[] logic in servicetalk-utils-internal

#862 is adding a new module for shared utils (`servicetalk-utils-internal`). This opens up the door for extracting and consolidating the `Buffer` -> `byte[]` logic which has been copy/pastaed around for the lack of a shared home.

Start by reviewing issue #862 and locating the existing Buffer -> byte[] implementations that are copied across the repository. Compare their behavior, then consolidate the shared logic in the servicetalk-utils-internal module and verify the affected tests still pass.

Written by the indexing model from the issue text.