Restore IsDebugEnabled check for logging

[WilliamBZA]

Re-adds the IsDebugEnabled check that was removed in #2775.

The explicit call to builder.SetMinimumLevel(loggingSettings.ToHostLogLevel()); sets the logging level correctly between the instances of NServiceBus.Extensions.Hosting.DeferredLogger and NServiceBus.Extensions.Hosting.Logger which means the fix for #2775 is no longer required.

[ramonsmits]

+1 for restoring the if(logger.IsDebugEnabled on the log entries that use string interpolation.

Regarding builder.SetMinimumLevel(loggingSettings.ToHostLogLevel()), that is required as AFAIK the NLog configuration is independant on the logging extensions. For example, NLog verbosity could be set to Trace, and logging extensions to Info meaning logger.Debug would not log anything.

Reason this is different for the DeferredLogger is that only at the start IsDebugEnabled likely returns true until the endpoint is started if NLog was configured to only log Info or more severe. The items temporarily stored in the deferredlogger instance will be flushed but none of them will be written.

Maybe @danielmarbach can comment on my deferred logger assumptions.

I'm not really sure how the NServiceBus hosting extensions work. I do think there is a code smell, the deferred logger instanced shouldn't be needed due the way how start tasks currently work it is required but IMHO should be improved. Currently you could for example not cache IsDebugEnabled in a readonly bool IsDebugEnabled = logger.IsDebugEnabledto JIT compile all theseif`'s from the runtime code.

Small comment on the PR, these 2 changes are independant (IsDebugEnabled vs init) and maybe consider 2 PR's?