Add support for connection key columns

[kaidaguerre]

a ConnectionKeyColumn defines a column that has a value which maps 1-1 to a Steampipe connection
and so can be used to filter connections when executing an aggregator query

These columns are treated as (optional) KeyColumns. This means they are taken into account in the query planning

• so it is more likely that the scans will be planned to use these as quals where possible.

When executing an aggregator query, if one of these columns is used as a qual, the execution code
will filter the list of connections by comparing the qual value with the column value returned by the
[ConnectionKeyColumn] Hydrate function.