43703: storage/batcheval/result: perform various cleanup on LocalResult struct r=nvanbenschoten a=nvanbenschoten

This PR contains a series of cleanups to the `result.LocalResult` struct. It leaves the structure in a good position to be extended to include information about newly written, updated, and removed intents, which are hooked into the `concurrency` package in future commits the same way that `UpdatedTxns` is currently hooked into the TxnWaitQueue.

The changes are broken into a series of incremental steps to make them easier to review in isolation.

43722: opt: fix assertion failure due to lax empty key r=RaduBerinde a=RaduBerinde

PR #43532 removed the concept of lax constant functional dependencies.
There is a left-over case when we downgrade a key: if we had a strong
empty key, the result is a lax empty key which is no longer a concept.

This change fixes this by removing the key altogether in this case.

Fixes #43651.

Release note (bug fix): fixes "expected constant FD to be strict"
internal error.

43734: pgwire: deflake TestAuthenticationAndHBARules r=knz a=knz

Fixes #43733.

This was my mistake to fix, I had forgotten that cluster settings
propagate asynchronously.

Release note: None

Co-authored-by: Nathan VanBenschoten <nvanbenschoten@gmail.com>
Co-authored-by: Radu Berinde <radu@cockroachlabs.com>
Co-authored-by: Raphael 'kena' Poss <knz@thaumogen.net>

43731: pgwire,hba: introduce the 'trust' and 'reject' auth methods r=knz a=knz

First commits from #43734 and #43726.

The 'trust' and 'reject' methods, as their name implies,
unconditionally allow and deny authentication of matching connections.

This patch introduces them as a prerequisite to later work on Unix
socket authentication, but also to introduce a bit of infrastructure
that binds new auth methods to a minimum required cluster
version. This new infrastructure ensures that future new auth methods
do not risk hosting a mixed-version cluster. (See discussion on #43717.)

The patch also introduces support for the 'local' rule prefix, which
is still unused.


Co-authored-by: Raphael 'kena' Poss <knz@thaumogen.net>