Add `_internal::UserAgentGenerator`

[antkmsft]

          Thinking beyond the release issue today (i.e. for a future change), does it make sense to move such a helper class to internal, if we expect SDK clients to use it?

Originally posted by @ahsonkhan in https://github.com/Azure/azure-sdk-for-cpp/pull/6063#discussion_r1786908445

          Yes, we should effectively create an alias in the `_internal` namespace, and use it in the new version of eventhubs. But we probably still have to keep this one in `_detail` forever, for the compatibility with previously released eventhubs.

Originally posted by @antkmsft in https://github.com/Azure/azure-sdk-for-cpp/pull/6063#discussion_r1786910291

[antkmsft]

Using a function from _detail could be considered a bug and therefore rightfully fail. I think we can move this from _detail to _internal, drop the deprecated function, make EventHubs to use the function from _internal, release the new GA of Core, release the new Beta of EventHubs with the bugfix.