feat: validate edge function signature at build time

[jackiewmacharia]

🎉 Thanks for sending this pull request! 🎉

Please make sure the title is clear and descriptive.

If you are fixing a typo or documentation, please skip these instructions.

Otherwise please fill in the sections below.

Which problem is this pull request solving?

Example: I'm always frustrated when [...]

List other issues or pull requests related to this problem

This fixes https://github.com/netlify/pod-compute/issues/249

Describe the solution you've chosen

Example: I've fixed this by [...]

Describe alternatives you've considered

Example: Another solution would be [...]

Checklist

Please add a x inside each checkbox:

• I have read the contribution guidelines.
• The status checks are successful (continuous integration). Those can be seen below.

A picture of a cute animal (not mandatory but encouraged)

[jackiewmacharia]

A couple of thoughts/pointers on this:

• I've added the default export validation to the config setup so we only run this block once. This ties this code to config export feature flag and options.
• I've set it to error on for default export related errors so as to fail at build time.
• There might be a way to separate validation from getFunctionConfig by moving this block and the config error logger to a validator util. If this is ok, I'm glad to do this in this PR.

[eduardoboucas]

I left a few small comments, but otherwise looking great!

[eduardoboucas]

🚀