Fixed length protocol: allow to define custom length detector

[heathercaldwell]

Prev ID#: OCTCR17I269506

Fixed length protocol requires a length to be specified in the incoming message, which is not enough to cover some use cases.

Requirements:

• Message has always A fixed length and this length is not present in the message.
• Messages are separated by A delimiter.
• Length of the message can be determined from the payload (message for Amex card will have different length then Visa etc.)

[Jakub]

First requirement resolved in 5.4: In case of constant message length, user can specify it in a Data column. Attributes should be set to '1' and all other columns should be left blank