fix: Ensure that Iterator and Readable work correctly for INSERT

[BobdenOs]

Currently a lot of the cds-dbs logic relies on non consuming Arrays which are required to be kept fully in memory. It should be allowed to INSERT any kind of Iterator for scenarios where it is possible to not load the whole dataset into the application.

Examples:

const { Books } = cds.entities

// Generator
const gen = function*(amount) {
  for(let i = 0; i < amount; i++) {
    yield { ID: cds.utils.uuid() }
  }
}
await INSERT(gen(100)).into(Books)

// Generator (async)
const genAsync = async function*(amount) {
  for(let i = 0; i < amount; i++) {
    yield { ID: await fetchKey() }
  }
}
await INSERT(genAsync(100)).into(Books)

// Readable (raw)
express.post('/upload', async (req,res) => {
  await INSERT(req).into(Books)
  res.end('done')
})

// Readable (object)
await INSERT(Readble.from(genAsync(100))).into(Books)