Envvars for Snowflake adapter

[mildbyte]

Add support for passing environment variables to the Snowflake adapter (e.g. HTTP_PROXY / HTTPS_PROXY / NO_PROXY etc).

Example:

$ sgr mount snowflake test_snowflake -o@- <<EOF
{
    "username": "username",
    "password": "password",
    "account": "account.region.vendor",
    "database": "SNOWFLAKE_SAMPLE_DATA",
    "schema": "TPCH_SF100",
    "envvars": {"HTTPS_PROXY": "https://proxy.company.com"}
}
EOF

Fixes #413