[BUG] Add default to fields that also have skip_serializing_if

[brooksmtownsend]

In the Metadata example: https://github.com/wasmCloud/wadm/blob/main/crates/wadm-types/src/lib.rs#L180 if you construct a manifest and try to deploy it using wadm-client, the annotations field will not be serialized, and then wadm will fail to deserialize the manifest.

For any of our BTreeMap / HashMaps that have the serde skip_serializing_if annotation, we also need to add default there (see labels for an example)

This is a great first issue to fix a frustrating bug.