API: pseudo-public internals API for downstream libraries
Downstream libraries accessing core.internals causes headaches pretty regularly  (https://github.com/pandas-dev/pandas/pull/40149#discussion_r585699450, #38134,).  The proposal: define a public-ish stable-ish API to expose for these libraries, leaving us free-er to make changes in internals.

This exposes `make_block`.  I expect we'll also need to expose one or both of `create_block_manager_from_arrays`/`create_block_manager_from_blocks` (comment in `internals.__init__` says they are there for downstream xref #33892)
