investigate migrating from mysql to postgres

[slingamn]

At least two operators reported that they currently use MySQL, but also maintain a Postgres for other software, and would like to consolidate in Postgres.

Some possibilities:

• pgloader supports transferring from MySQL to Postgres: develop and test a semi-supported workflow for using it. See the pgloader documentation: https://pgloader.readthedocs.io/en/latest/ref/mysql.html
• Some deployments might want to switch from MySQL to SQLite?

[slingamn]

pgloader appears to be hard incompatible with MySQL 8 (it doesn't handle caching_sha2_password or even switching between different password mechanisms).

[slingamn]

First pass attempt on #2373