Refactor inngestgo to remove inngest.* configuration references

[tonyhb]

We shouldn't have circular imports between OSS and this repo. When defining functions in an external lib, I should only need to import this library to configure functions.

EG:

inngestgo.FunctionOptions{
  Concurrency: &inngest.ConcurrencLimits{}
}

This is wrong.

We should:

1. Move function configurations into their own subpackage inngestgo/fns
2. Recreate function configuration within this package entirely
3. Consider ergonomic function configuration helpers, eg fns.FunctionConcurrency(1) as a helper for defining function limits overall.