[C++] uint8_t is not guaranteed to be a valid byte alias

### Describe the bug, including details regarding any error messages, version, and platform.

The standard states that accessing bytes of an object through a type other than `char`, `unsigned char`, or `std::byte` is undefined behavior. However in arrow we mostly use `uint8_t` when examining bytes. It'd be more correct to replace such occurrences with `std::byte`, and will guard against future surprises if (for example) a new optimization pass leverages this strict aliasing rule in a way that will break our code.

### Component(s)

C++