AzureServiceBusHealthCheck chooses the connection method based on the connection string being present

[tj-spyro]

What this PR does / why we need it:

• Using the connection string for Azure Service Bus is now based on it being not null or empty, instead of a credential being not null.
• Azure Service Bus health checks default to using the DefaultAzureCredential when a credential is not provided.
• Adds simple *WithNamespace() extension methods for registering using the DefaultAzureCredential and minimal configuration of the queue/topic/subscription when using a fully qualified namespace.

This aligns with the direction and behaviour the dotnet Aspire Team take for which options take precedence and default values.

Which issue(s) this PR fixes:
#2395

Special notes for your reviewer:
n/a

Does this PR introduce a user-facing change?:
This would not break existing users configuration.

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests
• Unit tests passing
• End-to-end tests passing
• Extended the documentation
• Provided sample for the feature

[tj-spyro]

@dotnet-policy-service agree