General JSON reading and using in Functions/matprop/etc

[GiudGiud]

Reason

As discussed in: RemDelaporteMathurin/h-transport-materials#108
we would like to be able to read coefficients from JSON databases. This is in support of T propagation modelling efforts in TMAP8, but is a very general need. We can have users store data in JSON format in a lot of different applications. in fact, there's already been some JSON loading use there: #20851

@simopier @cticenhour for info

Design

@dschwen if you could please advise us here considering your work on #20851

I would suggest following the PropertyFileReader design:

• Add a JSON reader user object that reads the file and has all the getters desired with some checks on sizes etc
• HEAVILY rely on nlohmann::json there
• this enables us to read the file only once and retrieve it from multiple objects
• a suite of FromJSON objects that can use it. The first and most-needed is a material property, but we could add a functor material too

Impact

Standardised JSON reading from the input file will provide more flexibility to users
Ease of use for others, not just CSV supported
Can help interfacing with other codes as well

[dschwen]

Not sure how much we'd need in addition to nlohmann::json. You might want to read the JSON file only on rank 0 and use MPI to broadcast the JSON tree to other ranks. There isn't any size checking that would be applicable here. If you want something along schema checking then use https://json-schema.org/ with a validator library.