Allow Different Types of Compute Other Than Lambda

Currently, it is assumed that lambda will be used as the source of compute for the `NextjsDistribution` construct. Instead of passing `serverFunction` and `imageOptFunction` directly into this construct, what if it was abstracted away and `serverHttpOrigin` and `imageOptOrigin` were passed into the construct such that other types of compute could be defined by user and passed in?

I likely won't have time to implement this, but said I would create this issue in #108. Wanted to create the issue in case someone else wanted to implement.