SQLAlchemy and Snowflake connectors

[mildbyte]

Add some machinery to support SQLAlchemy connectors via a customized version of the FDW (that allows users to get tables to point to subqueries on the remote database to get around pushdown limitations and syntax constructs / operations that aren't available in PostgreSQL).

As a showcase, ship a Snowflake data source in this PR:

$ sgr mount snowflake test_snowflake -o@- <<EOF
{
    "username": "username",
    "password": "password",
    "account": "acc-id.west-europe.azure",
    "database": "SNOWFLAKE_SAMPLE_DATA",
    "schema": "TPCH_SF100"
}
EOF

$ sgr mount snowflake test_snowflake_subquery -o@- <<EOF
{
    "username": "username",
    "password": "password",
    "account": "acc-id.west-europe.azure",
    "database": "SNOWFLAKE_SAMPLE_DATA",
    "tables": {
        "balances": {
            "schema": {
                "n_nation": "varchar",
                "segment": "varchar",
                "avg_balance": "numeric"
            }
            "options": {
                "subquery": "SELECT n_nation AS nation, c_mktsegment AS segment, AVG(c_acctbal) AS avg_balance FROM TPCH_SF100.customer c JOIN TPCH_SF100.nation n ON c_nationkey = n_nationkey"
            }
        }
    }
}
EOF