Usage of substrait extension types

[pdet]

We now verify the custom extensions defined in the extensions folder.

If they exist there, we properly add the extension to our plan and create a function signature in the format:
<function name>:<short_arg_type0>_<short_arg_type1>_..._<short_arg_typeN>

When consuming plans, we disregard this extra information. I guess it might only be useful in the context of executing functions rather than the operator, but I'm not yet certain.

I think the main thing missing is that there are no guarantees that we are not exporting functions that are either native to Substrait or defined in these extension YAML files. This might be something to explore in the future if a system that tries to consume DuckDB Plans actually needs these.

Finally, I've done a lot of fiddling with the auto-generated proto code to get type signatures but did not manage to obtain them. I'm currently getting these signatures by parsing debug info, which is pretty hacky. I will need to find a cleaner way of doing this.

Fixes: #65

As an example, a plan for the query:

select sin(a) from t

where t is:

create table t (a double)

And the sin function, now defined in the Substrait extensions, looks like this:

{
  "extensionUris": [
    {
      "extensionUriAnchor": 1,
      "uri": "https://github.com/substrait-io/substrait/blob/main/extensions/functions_arithmetic.yaml"
    }
  ],
  "extensions": [
    {
      "extensionFunction": {
        "functionAnchor": 1,
        "name": "sin:fp64"
      }
    }
  ],
...