feat: add unsafe Utf8Bytes::from_bytes_unchecked

[Totodore]

I'm in a case where I already have a Utf8Bytes equivalent in my own codebase (a simple Bytes wrapper), therefore I am sure that this [Bytes] is valid utf8 data and I want to convert it to a Utf8Bytes.

However there is currently now way to construct a Utf8Bytes without checking the encoding even if we are sure about it.

I propose to add the from_bytes_unchecked which is similar to String::from_utf8_unchecked.