[FIX]product:change pricelist fixed_price field from float to monetary

steps to reproduce:
-change decimal accuracy for "Product Price"
-create a price list with fixed price
-the fixed price field does not takes the decimal accuracy into account

this was working in V11 and V12
fixed_price was a float field like other price fields

previous behavior:
fixed_price is a monetary field
when setting up a fixed price for a pricelist, it takes into account
the precision from the current currency

current behavior:
fixed_price is a float field
when setting up a fixed price for a pricelist, it takes into account
the proper decimal accuracy setting ("Product Price")

opw-2121964

closes odoo/odoo#40777

Signed-off-by: Jorge Pinna Puissant (jpp) <jpp@odoo.com>