Support passing secrets via environment variables

[shamahov]

Is your feature request related to a problem? Please describe.
When we tried to add ClickHouse support to Spark-Connect, we faced the issue that it's not possible to keep secrets inaccessible to the client because they are passed via the configuration.

Describe the solution you'd like
When you pass property spark.sql.catalog.clickhouse.password_env_variable=<ENV_VARIABLE_NAME>, connector retrieves the password using the specified environment variable name