Auto merge of #103754 - SUPERCILEX:filled-mut, r=m-ou-se

Add back BorrowedBuf::filled_mut

This is useful if you want to do some processing on the bytes while still using the BorrowedBuf.

The API was removed in rust-lang/rust#97015 with no explanation. The RFC also has it as part of its API, so this just seems like a mistake: [RFC](https://rust-lang.github.io/rfcs/2930-read-buf.html#:~:text=inline%5D%0A%20%20%20%20pub%20fn-,filled_mut,-(%26mut%20self))

ACP: rust-lang/libs-team#139

Author: bors