parquet: reduce clone in delta byte array decoder

[lyang24]

Which issue does this PR close?

small optimization

Rationale for this change

key insight is the byte clone is cheap just a ref count compare to vec clone is a alloc + memcopy.

before

let mut result = Vec::new();          // alloc #1
result.extend_from_slice(prefix);
result.extend_from_slice(suffix);

let data = Bytes::from(result.clone()); // alloc #2 + memcpy
item.set_from_bytes(data);
self.previous_value = result;          // keep Vec

after

let mut result = Vec::with_capacity(prefix_len + suffix.len()); // alloc #1
result.extend_from_slice(&self.previous_value[..prefix_len]);
result.extend_from_slice(suffix);

let data = Bytes::from(result);       // no alloc, takes Vec buffer
item.set_from_bytes(data.clone());    // cheap refcount bump
self.previous_value = data;           // move, no alloc

What changes are included in this PR?

previous_value type changed to Bytes
preallocate result vec capacity.

Are these changes tested?

the existing test should pass

Are there any user-facing changes?

no