feature: enable migrate msg to be either a struct or an enum

[kakucodes]

Working through a go-codegen failures while trying to generate types for DAODAO's dao_core contract, the root cause seemed to be that go-codegen insists that MigrateMsg is a struct, however for many more mature contracts that is not the case and migration requires an enum so I did the small amount of work to make that allowable and was able to successfully codegen said contract.

[srdtrk]

Looks good. Will merge after workflows pass. Can you add the problematic schema file into the integration tests as well? All you need to do is drop the file here. And then add it's name here and here

[srdtrk]

Lgtm. Thanks so much for the contribution. There seems to be another issue with dao-dao-core contract. So we can add its test once that issue is understood and resolved