Add ts_optional to structs, rather than on individual struct fields

[dessalines]

Requirements

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Is your proposal related to a problem?

We currently have to have ts_optional directives on every single struct field. This is verbose and error-prone.

Describe the solution you'd like.

ts-rs recently merged a PR adding it at the struct level.

On ts-rs 's next release, use this.

Describe alternatives you've considered.

NA

Additional context

Aleph-Alpha/ts-rs#366