DPL: refactor ConfigParamRegistry to use C++20

[ktf]

Use constraints to make sure we can out-of-line known
types of parameter.

Remove the need to include Array2D.h when not used.

Reduce dependence on ptree, in particular moving the helper methods
out of line.

[github-actions]

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[alibuild]

Error while checking build/O2/fullCI for 48757bd at 2025-01-10 13:36:

## sw/BUILD/O2-latest/log
c++: error: unrecognized command-line option '--rtlib=compiler-rt'
c++: error: unrecognized command-line option '--rtlib=compiler-rt'
collect2: error: ld returned 1 exit status
ninja: build stopped: subcommand failed.

Full log here.

[alibuild]

Error while checking build/O2/fullCI for c4a029d at 2025-01-10 14:58:

## sw/BUILD/O2-latest/log
c++: error: unrecognized command-line option '--rtlib=compiler-rt'
c++: error: unrecognized command-line option '--rtlib=compiler-rt'
/sw/SOURCES/O2/13850-slc8_x86-64/0/Framework/Core/include/Framework/ConfigParamRegistry.h:96:44: error: cannot convert 'boost::property_tree::basic_ptree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> >' to 'long long int' in initialization
ninja: build stopped: subcommand failed.

Full log here.

[ktf]

macOS tested separately. Merging.