Fix Polars expr pushdown

[evertlammerts]

See #98

We have our own Polars IO plugin to create lazy polars dataframes. We try to push as many predicates down into DuckDB as we can, for which we try to map Polars expressions (including datatypes) to SQL expressions. To do this we depend on Polar's Expr.meta.serialize.

None of this is very stable. Polars IO source plugins are marked @unstable and Expr.meta.serialize says "Serialization is not stable across Polars versions".

In this case the problems seems to come from Polars requiring an explicit scale to be set for decimals (this pr). The serialized format seems to have changed into:

{
  "expr": {
    "Literal": {
      "Scalar": {
        "Decimal": [
          1,
          38, // This now includes the scale
          0
        ]
      }
    }
  },
  "dtype": {
    "Literal": {
      "Decimal": [
        38, // this was already there
        0
      ]
    }
  },
  "options": "Strict"
}

Interestingly, even if we explicitly set precision to e.g. 20, the relevant part of the serialized expression looks as follows:

{
  "expr": {
    "Literal": {
      "Scalar": {
        "Decimal": [
          1,
          38, // Still 38?
          0
        ]
      }
    }
  },
  "dtype": {
    "Literal": {
      "Decimal": [
        20, // This is correct
        0
      ]
    }
  },
  "options": "Strict"
}

This PR allows for both a 2 and 3 item list for decimals.