Allow a list of keys in the `by` option of the v2 version of the `join` operator

New feature

Currently the by option in the .join operator for typed workflows allows only a single key to be specified. This on it's own is a very useful feature and will greatly improve how we write pipelines. It would however also be helpful if this option could take a list of multiple keys and only join entries where all of these keys match. This would prevent pipeline developers from having to do a .map on both channels before joining them. It would also increase the readability of the pipeline as it's clear from the join operator what keys are used exactly (instead of one custom key created just for joining)

Use case

Joining channels with records that have multiple keys in common

Suggested implementation

ch1.join(ch2, by['id', 'type']) or similar

Start at the v2 typed-workflow .join operator and trace how its current single-key by option is parsed and matched. Review the existing join tests for one key, then add coverage showing that a list such as ['id', 'type'] joins entries only when every key matches; done means the multi-key form works without requiring preceding .map operations.