IByteStreamCnvSvc: add interface for ByteStreamCnvSvc

Add an interface for ByteStreamCnvSvc, which exposes the getFullEventAssembler method.

Implementation details:

• The getFullEventAssembler is split into a non-virtual templated method that deals with the concrete FullEventAssembler (FEA) construction. This method uses the virtual findFullEventAssembler and virtual storeFullEventAssembler that is implemented in the ByteStreamCnvSvc to retrieve and store the FEA.
• Also remove some typedefs to make the code easier to read.
• Until all clients are migrated the interface boilerplate of the concrete class is kept for backwards compatibility.

In a follow-up MR all clients will be migrated to IByteStreamCnvSvc.

Comments on the new IByteStreamCnvSvc would be very much appreciated: @gemmeren @mnovak @maszyman @akraszna @amete