convert: add new RefArray type signature

[benlorenz]

This seems to be needed for julia nightly, otherwise the CxxWrap tests error with:

basic_types: Error During Test at /home/pkgeval/.julia/packages/CxxWrap/66SRr/test/basic_types.jl:186
  Test threw exception
  Expression: BasicTypes.test_argv(Cint(length(argv)), argv) == join(argv)
  type RefArray has no field cpp_object
  Stacktrace:
   [1] getproperty
     @ Base ./Base.jl:37 [inlined]
   [2] unsafe_convert(to_type::Type{CxxPtr{CxxPtr{CxxChar}}}, x::Base.RefArray{Ptr{Int8}, Vector{Ptr{Int8}}, Vector{Any}})
     @ CxxWrap.CxxWrapCore ~/.julia/packages/CxxWrap/66SRr/src/CxxWrap.jl:271
   [3] test_argv(arg1::Int32, arg2::Vector{String})
     @ Main.BasicTypes ~/.julia/packages/CxxWrap/66SRr/src/CxxWrap.jl:624
   [4] macro expansion
     @ /opt/julia/share/julia/stdlib/v1.11/Test/src/Test.jl:676 [inlined]
   [5] macro expansion
     @ ~/.julia/packages/CxxWrap/66SRr/test/basic_types.jl:186 [inlined]
   [6] macro expansion
     @ /opt/julia/share/julia/stdlib/v1.11/Test/src/Test.jl:1598 [inlined]
   [7] top-level scope
     @ ~/.julia/packages/CxxWrap/66SRr/test/basic_types.jl:37

(from https://s3.amazonaws.com/julialang-reports/nanosoldier/pkgeval/by_date/2023-10/24/CxxWrap.primary.log, before libcxxwrap broke due to a different change, see JuliaInterop/libcxxwrap-julia#137)

This error seems to come from: https://github.com/JuliaLang/julia/pull/51764/files#diff-2331196f664261004e7d661fc57c6076578eeebc0bb8ef941e1c9f4215d72c90

Stepping through the code with two different julia version from before and after this change I got for before:

About to run: <(Base.unsafe_convert)(CxxPtr{CxxPtr{CxxChar}}, Base.RefArray{Ptr{Int8}, Vector{Ptr{Int8}}, Any}(Ptr{I...>

and after (note the Vector{Any} instead of Any as third type for RefArray):

About to run: <(Base.unsafe_convert)(CxxPtr{CxxPtr{CxxChar}}, Base.RefArray{Ptr{Int8}, Vector{Ptr{Int8}}, Vector{Any...>

And the new code then tried to call Base.unsafe_convert(to_type::Type{<:CxxBaseRef}, x) = to_type(x.cpp_object) instead of the one for RefArray.

Unfortunately the tests for this won't work for nightly due to the libcxxwrap error.
Edit: Locally this fixed the issue for julia nightly bde62adff7.