Move ZRC20 conversion rate from RateLimiterFlags to a separate structure

[lumtis]

Describe the Issue
The ZRC20 staking feature would require to use a conversion rate for ZRC20s. The first and simplest solution is to set this values from authority messages. Since we already store these values for the rate limiter, we can set these values into a general structure separated from the rate limiter flags. The rate limiter would reuse these values.

Expected Outcome
Remove ConversionRate from RateLimiter
Create a ZRC20ConversionRates structure in the authority module (since used by different unrelated module in the chai, looks the best place for it)

[lumtis]

No longer in consideration