Rollup merge of #140459 - niklasf:feature/read-buf-at, r=tgross35

Add `read_buf` equivalents for positioned reads

Adds the following items under the ~~`read_buf` (rust-lang/rust#78485)~~ `read_buf_at` (rust-lang/rust#140771) feature:

 - `std::os::unix::fs::FileExt::read_buf_at`
 - `std::os::unix::fs::FileExt::read_buf_exact_at`
 - `std::os::windows::fs::FileExt::seek_read_buf`

try-job: `x86_64-msvc*`
try-job: `test-various*`
try-job: `dist-various*`

Author: Zalathar