Use inline namespaces when generating code with the EXTRA_NAMESPACE option

The current EXTRA_NAMESPACE option behavior breaks the protobuf package consistency and disallow the use of the generated protobuf libraries without the knowledge of what EXTRA_NAMESPACE was used while generating the code.

The potential issue that namespace helps to solve - duplicated symbols at linker time, which still can be addressed by using inline namespace. We should generate the inline namespace in both scenario when EXTRA_NAMESPACE option is set and and it is not set. This won't avoid ambiguity when using messages from two different sources at compile time(which we now may treat as user failure), but prevents linker-time issue.