Allow iterating over batches of dataset items, instead of single items only

[gullmar]

E.g., add iterateBatches and greedyIterateBatches functions in ExtDatasetClient.

It would improve cases like this:

for await (const post of client.dataset<Post>(postsRun.defaultDatasetId).greedyIterate({
    offset: state.postsScraperRun.seenPosts,
})) {
    addPost(post);
    ++state.postsScraperRun.seenPosts;
}