Improve price fetching

[XavierAP]

AlphaVantage with free api key is reaching its limitations: when having more than five or so prices to fetch, often some fail. (Note: happening less lately, with six, app usage less frequent.)

Try another provider with free api key. Prefer if they can provide all prices in one call. (Fetching logic may need to change or be generalized.) So far found: https://marketstack.com/documentation#eod_data

Preferably keep the option to use different providers for different stocks (perhaps a prefix word in the fetchCode).