Implement form submission

[nicoburns]

Suggested implementation plan

• Modify Navigate event to take a Request rather than a Url
• Form submission should be triggered on:
  • Click of <input type="submit" />
  • Enter key press when single-line text-field is focussed
• Write a submission function that:
  • Iterates up the tree from the element that triggered the event looking for a <form> element (if no <form> element is found then do nothing)
  • Reads the action and method from the <form>
  • Searches the subtree under the <form> for input elements (we currently support: text, textarea, radio, checkbox, and hidden).
  • Composes a url and body from form attrs and input values.
  • Generates a Navigate event.

The initial implementation:

• Wouldn't necessarily need to support all types (maybe just text and hidden?)
• Wouldn't necessarily need to generate a "submit" event for Dioxus Native
• Could assume enctype of application/x-www-form-urlencoded. But should maybe check for multipart/form-data and do nothing / log warning if it is set.

Although of course these features could be implemented (although probably no point doing multipart encoding until we have file inputs)