Make functions aware of their app

[amh4r]

Description

This PR makes a handful of notable changes:

• Move the app ID to the client.
• CreateFunction accepts the client and returns an error.
• NewHandler accepts the client.
• Require function IDs to be explicit. Previously they were the slugified name.
• The default client no longer exists. Instead, users must create one with NewClient.
• Make Handler private.

These breaking changes are evident in this example file:
/~https://github.com/inngest/inngestgo/pull/92/files#diff-2bbb903e4dd6d4106cc1be9a7ba97c4f4d4a7eb08791721637e64bd30fd45718

Motivation

• Fixes step.Send only using DefaultClient.
• The compiler tells you that you need to create the client, rather than a runtime error telling you to set DefaultClient.
• step.Invoke doesn't need to be given the app ID (it's implicitly on the invoked function).
• Creating implicit function IDs by slugifying function names is problematic, as we found with v2 of the TypeScript SDK.

More generally, these changes align the Go SDK a little more with our other SDKs.