Core: Build partition spec with case insensitive source column identifiers

[findinpath]

Address the following scenario:

        onSpark().executeQuery(format(
                "CREATE TABLE %s (id INTEGER, `mIxEd_COL` STRING) USING ICEBERG",
                sparkTableName));
        assertQueryFailure(() -> onTrino().executeQuery("ALTER TABLE " + trinoTableName + " SET PROPERTIES partitioning = ARRAY['mIxEd_COL']"))
                .hasMessageContaining("Cannot find source column: mixed_col);

Trino lowercases internally the identifiers for the column names.

This PR attempts to provide a transparent fashion to allow building a PartitionSpec by specifying the source column names in a case insensitive fashion.