wip: add experimental openvpn-config endpoint

[ainghazal]

Add experimental entrypoint to provision credentials to OpenVPN experiment.

• Single provider (riseup) for the MVP.
• Append-only log for Clickhouse.
• Configurable "freshness" parameter to limit the query.
• Stub for inserting new credentials periodically.

[hellais]

I added some patches that ports this over to the new pattern.

It's unfinished, but it should give you an idea of what's needed to get it to work.

Main things missing are:

• [x] Remove any changes to the legacy API codebase to minimize impact
• [x] Adding the alembic DB migrations (I changed the pattern slightly to work with multiple services)
• [x] Finishing the response model to support all that you need (I suppose you also need to do something to discover IPs and put them in the config payload, but I didn't see that in the original code either)
• [x] Write tests for everything and reach at least 95% code coverage (the rest of the new API services have 100% code coverage)

[hellais]

Superseeded by: https://github.com/ooni/backend/pull/832