Support default values for annotated server_fn arguments

[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(#[server(default)] is_good: bool) -> Result<(), ServerFnError> {}

If is_good is absent in the request to the server API,
Default::default() is used instead.

This is the approach suggested by #1745 (comment)

[gbj]

Looks good, thanks! (Failing CI was due to my mistake on the main branch earlier, have tested locally and this works well.)