refactor(stf): find alternative to reflection.Set to achieve proto.Merge on STf

[testinginprod]

We cannot use proto.Merge in STF because it fails with custom types. So we're forced to use reflection.Set, which is a little hacky and might have impact in embedded environments.

Possible alternatives:

• marshal and unmarshal: performance would be bad.
• fix our gogoproto fork to have Merge work with custom types

Ref: https://github.com/cosmos/cosmos-sdk/blob/main/server/v2/stf/stf_router.go#L167