Allow mapping of column names when reading metadata from the external source

[chiaradiamarcelo]

We need this in cases where the representation of the column names in Exasol db are different than in the external source.

For example, ElasticSearch nested column names are case sensitive dot-separated names, as in book.author.name for example.
We cannot use this identifier in Exasol, as the dialect does not allow dot in quoted identifiers, thus "book.author.name" is not allowed.