* Fix NPEs in DSL Specs
The `BaseWsInboundGatewaySpec` and `TailAdapterSpec` don't override super methods
and when we call them we fail with NPE since `target` was not populated.
* Fix `BaseWsInboundGatewaySpec` and its inheritors to populate the `target`
from their ctors.
* Remove redundant methods from `BaseWsInboundGatewaySpec` hierarchy
* Propagate `AbstractWebServiceInboundGateway` properties directly to the target
from the `BaseWsInboundGatewaySpec` inheritors
* Override `MessageProducerSpec` methods in the `TailAdapterSpec`
to populate respective option into the `FileTailInboundChannelAdapterFactoryBean`
* Expose more missed options for producer endpoint in the `FileTailInboundChannelAdapterFactoryBean`
**Cherry-pick to `6.0.x` & `5.5.x`**
* * `acceptIfNotNull()` for `errorMessageStrategy` in the `FileTailInboundChannelAdapterFactoryBean`
* * Fix `Unmarshaller` population logic in the `MarshallingWsInboundGatewaySpec`
8dd1332d0a