Allow default values for absent server_fn argument deserialization

[g2p]

This allows form submission with checkbox inputs to work.
For example:

let doit = create_server_action::<DoItSFn>();
<ActionForm action=doit>
  <input type="checkbox" name="is_good" value="true"/>
  <input type="submit"/>
</ActionForm>

#[server(DoItSFn, "/api")]
pub async fn doit(is_good: bool) -> Result<(), ServerFnError> {}

Arguments absent in the request to the server API will use the Default::default() constructor.

Discussion

Maybe this could be restricted to only certain types; at the moment bool and String need it (for checkboxes and radio buttons) and I don't see a way to customize the (de)serialisation.
Another attempted workaround was to add
<input type"hidden" name="is_good" value="false"/>
immediately before the checkbox; serde refuses to deserialize a field that appears twice but that could probably be tweaked as well.

[gbj]

I would assume that adding a blanket #[serde(default)] like this breaks every type that doesn't implement Default — is that incorrect?

A better approach might be to add an attribute to arguments that could be passed through to serde, in the same way that the #[component] macro's #[prop(optional)] passes an attribute through to typed-builder.

[g2p]

Good points, it was a little more involved but I have #1762 which does this for server function arguments carrying a #[server(default)] annotation.