BE link error injection command

To enable the full range of link tests we should add a command for injecting errors, equivalent to empbutler do MY_DEVICE mgts inject-error

It should have three parameters:

• The channel IDs: ids (a string)
  • Though the command only act on the subset of requested Tx ports which are present not masked (like the configure command)
• Number of errors to inject: count
• The type of error to inject: error-type
  • A string, restricted to a specific set of values (using core::rules::IsAmong like in the Reset command). E.g. prbs, crc, index, control-word-type-single-bitflip, control-word-type-double-bitflip, header-single-bitflip, header-double-bitflip (or shorter values if we can think of them).