[Feature] Implement Lazy Migration from LegacyDec to Dec

[samricotta]

Recently we upstreamed the GDA decimal type https://github.com/cockroachdb/apd in this pr #20085.

What this issue entails is implementing a lazy migration strategy to transition from LegacyDec to Dec to handle differences in precision and state compatibility introduced by the new GDA decimal type from apd.

There were a few ideas around how to move forward but the most agreed upon was a lazy migration. This could work by using a character prefix to distinguish between the old and new data formats and from there only when data is accessed or modified it will be converted. This could be done in the Marshal and Unmarshal functions within Dec and LegacyDec but we also have AltValueCodec from collections, which could potentially be used.

[tac0turtle]

no need for migrations for legacydec to dec, closing this issue