fix(js-sdk): allow passing custom request body parameters in auth.register method (#12545)

The `auth.login` method of the JS SDK allows passing custom, which is useful for custom authentication providers. For example:

```ts
const response = await sdk.auth.login("customer", "phone-auth", {
      phone
    })
```

However, the `auth.register` method doesn't allow that, so we can't do the following:

```ts
const response = await sdk.auth.register("customer", "phone-auth", {
      phone
    })
```

Instead, we'd have to use the `client.fetch` method.

This PR fixes the input type of the payload passed to the `register` method to be similar to that of `login`,  which would allow using it with custom authentication providers

.changeset/beige-carrots-care.md

@@ -0,0 +1,5 @@
+---
+"@medusajs/js-sdk": patch
+---
+
+fix(js-sdk): allow passing custom request body parameters in auth.register method