Add MySQL support to River - #1227

Here, add a new driver `rivermysql` that brings MySQL support to River.
Similar to SQLite, it's unfortunately not quite as good as the Postgres
driver, but it does the job. MySQL has more facilities than SQLite, but
it's still missing some major niceties like `RETURNING`, which for many
queries requires us to write an implementation using two operations --
one that performs the action, and then another that loads back the
result using returned IDs. It also doesn't have listen/notify.
Luckily, _like_ SQLite, any of this nastiness stays cordoned to the
driver layer and doesn't leak into the mainline River code. We've got
good driver testing and basic tests on clients for each driver in place
so we get reasonable assurance that everything works.
In the before times, I would've been fairly concerned at the additional
maintenance burden that supporting another database would bring, but
with the rise of LLMs I think it's more plausible that we can bring
something like this in without much trouble. Incredibly, I was able to
get almost all of this implemented in just one evening whereby my SQLite
driver took me multiple months.
I don't want to bring MySQL in as a hard dependency, so I've made the
MySQL tests disabled by default. Use `RIVER_MYSQL_TESTS_ENABLED=true` to
activate them. Only one CI matrix case runs MySQL tests so that we don't
have to repeat them to exhaustion on every version of Go and Postgres.
See also:
#158