feat(proto): Multi-table and as-of joining

[rtpsw]

We'd like to add support for multi-table and as-of joining. Multi-table means multiple right tables, i.e., extending to left-join-right(s). As-of joining uses an ordered key with inexact matching - it is a time-series operation.

[westonpace]

If we'd like to continue then I think this issue should be split into one issue for each part of the ask.