fix: dictionary decimal vector optimization

[kazuyukitanimura]

Which issue does this PR close?

Part of #679 and #670
Related #490

Rationale for this change

For dictionary decimal vectors, it was unpacking even for Int and Long decimals that used more memory than necessary.

What changes are included in this PR?

Unpack only for Decimal 128

How are these changes tested?

Existing test

[kazuyukitanimura]

@andygrove @parthchandra I moved the logic to the reader side, in that way, this PR will help for native exec as well

[parthchandra]

I'm not entirely sure how this helps on the exec side, could you provide a short explanation?
Otherwise lgtm

[parthchandra]

I wonder if this need to be made thread safe?

[kazuyukitanimura]

I don't think this gets read in multiple threads, but even if it would be, it is overwrite, so should be fine.

[kazuyukitanimura]

I'm not entirely sure how this helps on the exec side, could you provide a short explanation? Otherwise lgtm

Thanks @parthchandra
When the native exec is enabled CometDictionary is still called that decode the dictionary. However the native exec does not use CometDictionary to read the values. So by moving the logic to the reader, it decodes only when we need it (scan-only case)

[kazuyukitanimura]

Merged Thanks @parthchandra @andygrove