Make adding a computed column for viewing message body an opt-in

[SzymonPobiega]

The PR adding a computed column has been merged to develop, however, it should be a configurable opt-in. The goal is to add methods that allow to turn it on, it will be disabled by default.

• Add info in doco (the feature hasn't been documented yet) Particular/docs.particular.net@e1ee79a

Based on this comment we might need to make this feature configurable to account for BOM or different encodings.

[SimonCropp]

so in sql native i allowed people to override it https://github.com/NServiceBusExtensions/NServiceBus.Native/blob/288e507e930a228e0d858d6d722b1a13816d5d7e/SqlServer.Native/BaseQ/BaseQueueManager_QueueCreator.cs#L34