Implemented basic Price FieldType.

> Part of [EZP-23034](https://jira.ez.no/browse/EZP-23034)

Basic implementation, based on Float. No external storage yet. SPI & Unit tests are there.

Manual VAT handling is being worked on in #14
remark: I know this comes from legacy, but this is not very accurate if we have prices with 3 (or more?) digit (think fuel price in France for instance).