Implement currency history

[lparolari]

• [ ] Move out dollar_rate from Currency entity
• [ ] Replace dollar_rate filed on Currency with a custom field responsible to fetch latest dollar rate available
• [ ] Update the database initialization and first value fetch (see note below)
• [ ] Track dollar_rate history with a job
• [ ] Write a document keeping track of all jobs executing on server
• [ ] Expose endpoints with aggregate details (take inspiration from nomics)

The dollar_rate filed should be moved out to a new entity which is in charge of tracking currency history. The field on the original entity may become a custom fields which is instantiated to the latest available value.

Please note that without values (no history for a given currency) all the system breaks. We need to enforce default values on init and then fetch new ones.

[lparolari]

Well, there are some services that given a point in time and the currency name can return currency value. Thus, it's not convenient to implement this feature.