Add Frankfurter v2 provider (frankfurter_v2 driver)

[anousss007]

Summary

The bundled frankfurter driver targets the v1 API (GET /latest?from=&to= returning { "rates": { "CODE": rate } }). The v2 API is shaped differently:

• endpoint: GET /v2/rates?base=EUR&quotes=USD,AED
• response: a list of rows — [{ "date": ..., "base": ..., "quote": ..., "rate": ... }]

So v2 can't be reached by swapping FRANKFURTER_BASE_URL alone (hitting /v2/latest?from=... returns a 404). v2 also covers a much larger currency set than the ECB-only v1 — for example AED and MAD, which don't come back from the v1 latest call — and that's the main reason to support it.

What this adds

• FrankfurterV2Provider — calls /rates with base/quotes and maps the v2 rows back into the quote => rate shape before building Rates.
• A new frankfurter_v2 driver in ExchangeRateManager (no API key; base URL configurable via FRANKFURTER_V2_BASE_URL, defaulting to https://api.frankfurter.dev/v2).
• Config block, README section, unit tests, and the manager "can instantiate all drivers" dataset entry.

It's kept as a separate driver (frankfurter_v2) rather than changing frankfurter, so it's fully non-breaking. Happy to instead fold it into the existing driver behind a version option if you'd prefer that shape.

Small note: the provider parses the response date with !Y-m-d, so the retrievedAt date can't roll over when the instance is converted to the Europe/Amsterdam timezone (independent of the runner's local time/zone).

Follows up on the discussion about adding v2 support.