Length-related Functions Should Return Unsigned Integer

[austin362667]

Describe the bug

The string array functions length() and bit_length() should not return length as signed integer.

To Reproduce

See

arrow-rs/arrow-string/src/length.rs

Line 47 in 5ad621f

/// Returns an array of Int32/Int64 denoting the length of each value in the array.

and

arrow-rs/arrow-string/src/length.rs

Line 111 in 5ad621f

/// Returns an array of Int32/Int64 denoting the number of bits in each value in the array.

Expected behavior

Should return unsigned integer, since there are no negative length in our context.

Additional context

Thanks for @alamb bringing this up in the discussion on #6671 (comment)

Note: The actual Arrow spec uses integer types.

[tustvold]

I think we probably want to get some broader thoughts on this, whilst I agree unsigned is likely more appropriate in the Rust ecosystem, lots of systems, arrow included, prefer signed quantities for compatibility reasons (see StringArray, ListArray, etc...)

Either way, this isn't a bug and would be a breaking change

[alamb]

Specifically the Arrow spec uses i32/i64 typically for lengths for compatibility with Java (which does not support unsigned integers 🤷 )

[austin362667]

So I think maybe we can close this issue?