Adding DLL export/import tags to generated public upb API (redux) (#17079)

[copybara-service]

Adding DLL export/import tags to generated public upb API (redux) (#17079)

Picking up #14981 from @dawidcha after several months of radio silence.

Quoting the OP of that PR:

I have been collaborating with the grpc developers to make it possible to build that library as a Windows DLL - a couple of PRs were already merged, more like grpc/grpc#34345 are pending.

The grpc library incorporates some upb-generated, and upbdefs-generated code into grpc.dll, which is referenced by other code that consumes the library. Since this is now a DLL, that code doesn't know how to link to these generated symbols because they are not annotated with __declspec(dllimport).

This PR aims to fix that by introducing a parameter 'dllexport_tag' to the upb and upbdefs plugins. That parameter should be a string e.g. MYAPP_DLL and when set, the extern symbols are annotated with a macro with that name. This can either be set externally to __declspec(dllimport) or, as is usual practice, when compiling code into a DLL, the macro <dllexport_tag>_EXPORT (i.e. MYAPP_DLL_EXPORT) is defined, and when consuming the DLL <dllexport_tag>_IMPORT is defined if neither are defined then the MYAPP_DLL macro becomes empty string which is what you want for building a static library.

This is a continuation of #14230

Fixes: #14255

Towards #13726
Closes #14981

Closes #17079

COPYBARA_INTEGRATE_REVIEW=#17079 from h-vetinari:add_dll_tags 34927b1
FUTURE_COPYBARA_INTEGRATE_REVIEW=#17079 from h-vetinari:add_dll_tags 34927b1