Add price feed

[ikripaka]

Describe the feature

The Price Feed feature adds the ability to discover or retrieve the current BTC/USDT spot price on Liquid network.

For example, like this:
./simplicity-dex price-feed - to retrieve only BTC/USDT pair in Liquid
./simplicity-dex price-feed -pair ETH/USDT - to retrieve only ETH/USDT pair in Liquid

[zahorodnyi]

I implemented a very simple version of this feature (in feature/price-feed);
it supports only the BTC/USD price feed. I am not sure if we'll need this feature to show other currency pairs like ETH/EUR, or responses from other exchanges, but the code can be expanded to support this.

[zahorodnyi]

I improved the feature with a few small updates.

For now, there’s one problem: we can make only about 3 requests per minute (otherwise we get a rate-limit error). I don’t think this is a big deal, because we don’t need to make an API request every time we need the price. Instead, we can store the price and its timestamp somewhere and make a request only if the last timestamp is no longer up to date.

The reason is that the price we get from the exchange changes only once every 30-60 seconds (we should confirm the exact interval), so 3 requests per minute is enough.

P.S. We need the timestamp to check whether the last price is still up to date.