feat(buffer): allow Buffer::into_mutable to return for sliced buffer from the middle and add Buffer::is_sliced helper

[rluvaton]

[!INFO] I'm modifying the code to remove a bug that we have here

Which issue does this PR close?

N/A

Rationale for this change

before, Buffer::into_mutable would panic if you pass it Buffer that was offseted from the underlying bytes, but it would be ok for sliced Buffer that start at the same position but ends before the data_ptr itself, so the mutable buffer had data that was outside the slice in that case.

this would allow sliced buffer to be converted to MutableBuffer.

if they don't want to convert to MutableBuffer for sliced Buffer they can call the new is_sliced function before.

What changes are included in this PR?

added Buffer::is_sliced function and remove the panic if calling Buffer::into_mutable on owned sliced (where data_ptr is not ptr in the Buffer) Buffer

Are these changes tested?

Yes

Are there any user-facing changes?

Yes, new function and allowed for converting the entire buffer including data outside the slice to mutable

[rluvaton]

Created new pr instead:

• fix: disallow sliced buffer regardless of sliced direction in Buffer::into_mutable and add Buffer::is_sliced and Buffer::into_mutable_unsliced #10118