Add databases support

[melizeche]

Feature Request

Summary

Add database support, right now there's no historic data about the exchange rate

Basic example

Could be any SQL database but probably the best idea would be to use some ORM to support many databases at once, suggestions are welcome

Motivation

To eventually build graphs about the exchange rate evolution

[goodgodth]

I send PR already in #35 , let check it out

[fermellone]

1. I plan to use SQLModel as ORM, with PostgreSQL and SQLite as the base.
2. To create a decorator that stores the scrapped data within each defined database based on a schedule. (I thought on a daily schedule, but @melizeche mentioned it should be every 10 minutes.)
3. To implement the decorator to each scrapper function at coti.py.

Which fields should the "exchange_rates" table have?
Are there other tables that we should consider?