using strict custom type for coin tickers

[onur-ozkan]

Right now String types are used everywhere for coin tickers, and in most places there's no clear validation for them. Even if validation is done, it's unclear where or how it's happening. Using a wrapper type for coin tickers would be much more appropriate.

Just a simple/quick ref for the wrapper type:

struct CoinTicker(String);

impl CoinTicker {
    fn new(ticker: String) -> Result<Self, Error> {
        // various validations
        Ok(Self(ticker))
    }
}

This way, by default, every ticker used in KDF will be enforced to be validated.