Allow API client to provide serial and detect duplicate entry error

[pboguslawski]

This mod adds access to ClientProvidesSerialNumbers cert profile field from JSON profile config with client_provides_serial_numbers option. When enabled, cert serial must be provided by API client in request.

This mod adds also specific API error code in case of sign request failure caused by already taken serial number. For MySQL/MariaDB and SQLite only (to be done separately for PostgreSQL by devs using this DB engine).

Author-Change-Id: IB#1137304