Devise a migration plan to merge codecs with coreth 

We should also devise a migration plan to merge codecs with Coreth. linearcodec supports custom tags (see [this](https://github.com/ava-labs/avalanchego/blob/master/snow/engine/avalanche/vertex/stateless_vertex.go#L100) as an example).

_Originally posted by @ceyonur in https://github.com/ava-labs/subnet-evm/pull/1815#discussion_r2518415485_
            