feat: support callable parameters

[AayushSabharwal]

Close #2910

Requires JuliaSymbolics/Symbolics.jl#1270 and JuliaSymbolics/SymbolicUtils.jl#648

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[AayushSabharwal]

Draft while the issues outlined in the root comment are being worked on

[cstjean]

If I introduce

@parameters begin
     fn(::Real) = _f1   # taken from this PR's test
end

inside of my @mtkmodel, I get

AssertionError: Multiple independent variables are used in the model

Is that expected? Callable parameters are not yet supported inside @mtkmodel?

[AayushSabharwal]

Yeah @mtkmodel uses its own variable parsing for some reason. Open an issue? I'll try and figure this out.