Common: BigDecimal instead of Float for numeric EnrichedEvent fields

[istreeter]

Starting from Enrich version 3, we changed certain fields of EnrichedEvent from a String to a Float. It was part of the feature where we started validating that events conform to the atomic event iglu schema, see #517

The affected fields were:

• se_value
• tr_total
• tr_tax
• tr_shipping
• ti_price
• tr_total_base
• tr_tax_base
• tr_shipping_base
• ti_price_base

But, Float is not the best representation of these fields because of rounding errors. For example, if the incoming event has string value 137777104559 then it gets rounded to 137777104000 when converting to a Float.

Instead, we should handle these fields as BigDecimals