Remove /custom subpath for endpoints and add a way to customize the endpoint subpath

[nickrum]

Notice: This drops the /custom/ prefix for custom endpoint modules. Please update your projects logic if you rely on custom endpoints.

---

When using

export default router => {
  ...
};

to register a custom endpoint, it will now be registered at /<extension-name>.

This adds another way to register an endpoint:

export default {
  id: 'some-id',
  handler: router => {
    ...
  }
};

This endpoint will be registered at /some-id.

[joselcvarela]

What were the motivation for this?

I can see many users complaining about their custom routes not working because they are registering an already in use endpoint which is causing conflicts.

[tylerforesthauser]

If this PR allows one to nest custom endpoints in directories, the simple solution would be to structure your directory like endpoints/custom/my-endpoint/index.js.

[rijkvanzanten]

@joselcvarela You're right about that, and this is the sort of breaking change that we really don't want to make.. Once v9 is released in production. The reason we're still in RC right now is to make sure we get all these last (small) breaking changes out of the way before we're locked in