Parameter type enhancements for strings

I have a rather basic need to pass strings in between the cores via RPC, which actually represent a JSON.

I know those changes are probably meant in other repositories, but the use-case is in the context of `RPC`.

```
/Users/razvan/Library/Arduino15/packages/arduino/hardware/mbed_portenta/4.1.5/libraries/rpclib/src/rpc/msgpack/v1/object.hpp:213:7:
error: 'class arduino::String' has no member named 'msgpack_unpack'
v.msgpack_unpack(o.convert());
~~^~~~~~~~~~~~~~
```
and
```
/Users/razvan/Library/Arduino15/packages/arduino/hardware/mbed_portenta/4.1.5/libraries/rpclib/src/rpc/msgpack/v1/object.hpp:213:7:
error: request for member 'msgpack_unpack' in 'v', which is of non-class type 'const char'
v.msgpack_unpack(o.convert());
~~^~~~~~~~~~~~~~
```

It compiles with `std::string` tho.

Start with the RPC MessagePack conversion path at rpc/msgpack/v1/object.hpp, especially the msgpack_unpack call shown in the compiler errors. Reproduce the failures for arduino::String and const char, then compare them with the std::string case. Done means the requested string parameters can be passed through RPC without these compilation errors.

Written by the indexing model from the issue text.