Create API for financial_trxn, entity_financial_trxn tables

Create standard thin CRUD API including tests for the civicrm_financial_trxn and civicrm_entity_financial_trxn tables. This opens up the door for messing up accounting entries. Please add warning text in API explorer and in code implementing API and in tests documenting their use for create, update and delete:
"Given the importance of auditability, extension developers are strongly discouraged from writing code to add, update or delete entries in the civicrm_financial_item, civicrm_entity_financial_trxn, and civicrm_financial_trxn tables. Before publishing an extension on civicrm.org that does any of this, please ask for a special bookkeeping code review for the extension."

Once a 'thick' contribution API is created this message should be changed to make reference to it as the preferred method for extension code to change the data in those tables.