refactor: consider breaking out gas-prices config value into two different values

[jtieri]

Currently the relayer has a config value, gas-prices, that is a string representing both the gas price as well as the token denom. I propose that we break this value into two config fields, gas-price and gas-token-denom.

In its current state if you want to work with just the gas price or just the token denom you have to do some parsing of this string that involves regex and imo introduces an unnecessary area where bugs may occur. Breaking the existing config field into two fields will make working with chains that support dynamic fee markets easier.

This would be a breaking change to existing configs though so would need to be included in a major release.