Adds instruction to invoke a function via its url

[johnpc]

Description of changes:

This PR updates the Set up a Function page to include instructions on how to to invoke the function you've created via a function url.

We may prefer to edit this to put the function behind the data schema instead, but this option works. It also fixes the existing type issue in #7594 because it is able to type the function event as a LambdaFunctionURLEvent.

Related GitHub issue #, if available:

Fixes #7594

Checks

• Does this PR conform to the styleguide?

• Does this PR include filetypes other than markdown or images? Please add or update unit tests accordingly.

• Are any files being deleted with this PR? If so, have the needed redirects been created?

• Are all links in MDX files using the MDX link syntax rather than HTML link syntax?
  
  ref: MDX: [link](https://docs.amplify.aws/)
  HTML: <a href="https://docs.amplify.aws/">link</a>

When this PR is ready to merge, please check the box below

• Ready to merge

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[renebrandel]

this shouldn't be the "primary flow", as it'll likely result in a few unexpected situations for the customer. There's no authZ protection, e2e type safety, and customers need to think about "CORS".

I think we should this "Enable function URLs" as a separate with a callout that we don't recommend this directly.

[renebrandel]

Instead the default flow, should go through data.