Currency conversion enrichment: make the type of rate to use configurable

[benjben]

The config of the currency conversion enrichment exposes rateAt but it is never used.

Under the hood, the enrichment uses scala-forex (which is based on Open Exchange).

The API allows several types of conversion:

• With live rate
• With near-live rate
• With latest-prior EOD rate
• With latest-post EOD rate
• With specific EOD rate

The type of rate being used depends on:

• ForexConfig
• The API function being called

In Enrich it's always the same function that is used (with default EodRoundDown), which means that we can only use latest-prior EOD rate.

It would be great to update the enrichment to make it possible to decide which type of rate to use based on rateAt. Although not strictly required, the proper way would be to create a new schema with the different values in an enum.

At the moment the enrichment works only for some hard coded fields. It would be great to take the opportunity to make the list of fields configurable (e.g. like here).