[cosmwasm-schema] make deny_unknown_fields optional

[lvn-ryu]

See #2017 (comment) for suggested implementation

[webmaster128]

@aumetra could you have a look into that?

I think we should ensure NOT to set deny_unknown_fields for the deserializer. If we need to make modufications to the JSON Schema, those should be done in post-processing after schemars. Making undesired deserializer configurations for the sale of better TS types is the wrong priorization.

We'd need to revert parts of #1307 and move the removal of {[k: string]: unknown} in TypeScript to a later point in the process.