Add startup lifecycle hook

[inlined]

Adds a new function that can be used at global scope to perform initialization in production that won't be called during discovery. In this function, params will be guaranteed to have actual values.

Usage:

const apiKey = params.defineSecret("API_KEY");

onInit(() => {
  someSDK.init(apiKey.value);
});

[blidd-google]

LGTM

[blidd-google]

Do we leave globals as camel-cased or in CAPS?

[inlined]

Typically CAPS is for consts, which these aren't