append_many for Dictionary builders

[adriangb]

I have some data that I know is very repeated because it's essentially denormalized data, so the denormalized column is repeated for many rows. Because of this I'm using a dictionary, but I find myself forced into calling append_value in a loop to add the value multiple times. Under the hood this is going to hash it, find that it exists and append to the keys. I'd like a way to skip the extra hashing and instead add the value repeated n times.

I realize this can also be done via run encoding but I think this would be a simple and flexible API to add for dictionaries.

[alamb]

label_issue.py automatically added labels {'arrow'} from #6534