fdw/jdbc: Support and document adding additional jdbc drivers

[mfussenegger]

Problem Statement

CrateDB only includes a PostgreSQL JDBC driver. FDW/JDBC can't be used to connect to other servers like MySQL or MSSQL.
It's not documented how a driver for a different server can be added.

There should also be a way to add drivers for docker/container deployments. Maybe via a dedicated external-libs folder and volume mounts.

Possible Solutions

Some options:

• Document that jars can be placed into lib/
• Add separate external-libs/ folder and setup volume mounts for the docker image. See if adding class-loader isolation is necessary to avoid JarHell if the dependency include duplicate classes.
• Bundle more clients
• Offer option to bundle clients on demand/download

Whatever option is taken, we also need to reconsider the "local" protection limitations as pointed out in #15776 (review)
(Worst case option could be something like changing fdw.allow_local to support never | if-unkown | always), where if-unknown is strict for the PostgreSQL case or if the URL parses as URI, but accepts it if it can't parse the URI safely.)

Considered Alternatives

No response