currencyrate: propagate http errors to currencyrate rpc if a source is provided

specifically coindesk was constantly hitting API rate limits causing our tests to fail
so lets unit tests all endpoints with a snapshot of real responses and only allow for http
error 429 in integration tests

Also fix a flake in test_bkpr_currencyrate_persisted that would pick up a cached rate from CLN's own caching

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Author: daywalker90