Implement different strategy for resolving specific endpoint namespaces

[yvesgoeleven]

Right now they are defined on the message endpoint mappings, but to align more with the other transports this should be swapped out to use the connectionstrings section.

Also make this pluggable, so that users can define their own strategies. F.e. based on a header switch between europe and asia f.e.

[particularbot]

moved to https://github.com/Particular/NServiceBus.AzureServiceBus/issues/4