add deduction guides for Struct from std::tuple (Fix for C++23 on gcc 15)

[heinemml]

C++23 changed the generation of implicit deduction guides. This causes the compiler to also see deduction guides for std::tuple as candidates for sdbus::Struct.
Because of the competing guides the compiler doesn't know which one to pick.

This seems to be implemented from gcc 15 on and is thus causing breakage there.

To fix this we need to add explicit deduction guides when std::tuple is passed to sdbus::Struct.

fixes #524

[sangelovic]

@heinemml Thanks for reporting the issue and the PR. I'm fine with that except of a little detail -- we need to remove std::decay_t<> wrapper from the deduction guides. Keeping it would induce an unnecessary performance inefficiency on serialization implementation in SDBUSCPP_REGISTER_STRUCT macro, and would completely break deserialization implementation in that macro. The mechanism in those implementations is built on std::forward_as_tuple which creates a tuple of references, and hence we need to also create the same Struct with its element types being references. The result is an thin, "observer" struct which merely points to items in the user-defined struct, bringing the convenience of avoiding copies in serialization and making the deserialization happen through Struct instance into the destination user-defined struct. I can to the correction myself, or will you?

[sangelovic]

I already did it.

[sangelovic]

Unit tests would have failed on the original std::decay_t-based solution -- a symptom of broken user-defined struct deserialization:

1: [ RUN      ] AMessage.CanCarryUserDefinedStruct
1: /home/projects/Kistler-Group/sdbus-cpp/tests/unittests/Message_test.cpp:535: Failure
1: Value of: dataRead
1: Expected: is equal to 72-byte object <FE-18 36-00 00-00 00-00 30-1A B1-9F 99-55 00-00 05-00 00-00 00-00 00-00 68-65 6C-6C 6F-00 00-00 02-00 00-00 00-00 00-00 60-2A B1-9F 99-55 00-00 90-D9 B0-9F 99-55 00-00 02-00 00-00 00-00 00-00 01-00 00-00 65-2F 6F-62>
1:   Actual: 72-byte object <00-00 00-00 00-00 00-00 78-8C E1-30 FC-7F 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 88-8C E1-30 FC-7F 00-00 88-8C E1-30 FC-7F 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00 00-00> (of type my::Struct)
1: 
1: [  FAILED  ] AMessage.CanCarryUserDefinedStruct (0 ms)