[ArrayManager] Implement concat with axis=1 (merge/join)
The (hopefully) "easy" part of https://github.com/pandas-dev/pandas/pull/39612 for which there seems less discussion: this separates the pieces from https://github.com/pandas-dev/pandas/pull/39612 that are needed to get `concat` with `axis=1` working (`concat_axis=0`), which at least enables the join / merge usecases.