feat: Allow functionAssociations overrides

[coryasilva]

Fixes #229 Allows functionAssociations to be overriden by exposing an optional overrides.nextjsDistribution.viewerRequestFunctionProps.code prop. Sample usage:

const cmsRedirectKvStore = new cloudfront.KeyValueStore(this, "CmsRedirectKeyValueStore");

const viewerRequestCode = `
import cf from 'cloudfront';

const kvsId = 'KEY_VALUE_STORE_ID_PLACEHOLDER';
const kvs = cf.kvs(kvsId);

async function handler(event) {
  // My custom code to do canonical redirects here...
  // Or my code to use kv store here...

  // INJECT_CLOUDFRONT_FUNCTION_HOST_HEADER

  // INJECT_CLOUDFRONT_FUNCTION_CACHE_HEADER_KEY
}
`

viewerRequestCode.replace(
  'KEY_VALUE_STORE_ID_PLACEHOLDER',
  cmsRedirectKvStore.keyValueStoreId,
);

const nextjs = new Nextjs(this, 'Nextjs', {
  nextjsPath: './web',
  overrides: {
    nextjsDistribution: {
      viewerRequestFunctionProps: {
        code: cloudfront.FunctionCode.fromInline(viewerRequestCode)
        keyValueStore: cmsRedirectKvStore,
      },
    },
  },
});

Also enable better cache strategy stolen from SST impl

This does not require but compliments the efforts for #234 and works towards resolving part of discussion #222