feat: FixedSizeBinaryArray::value_data return reference

[wjones127]

Which issue does this PR close?

Closes #4820.

Rationale for this change

Make it possible to get reference to underlying values.

What changes are included in this PR?

Are there any user-facing changes?

This is a change to the API. Existing users calling .value_data() will need to rewrite as .values().clone().

[tustvold]

Perhaps we should be consistent with GenericByteArray and have

/// Returns the values of this array
///
/// Unlike [`Self::value_data`] this returns the [`Buffer`]
/// allowing for zero-copy cloning
#[inline]
pub fn values(&self) -> &Buffer {
    &self.value_data
}

/// Returns the raw value data
pub fn value_data(&self) -> &[u8] {
    self.value_data.as_slice()
}