Schemachange

Changed
- Added support for unencrypted private keys
- `SNOWFLAKE_PRIVATE_KEY_PASSPHRASE` environment variable is no longer required if the key is not encrypted

Changed
- Added 'schemachange' as an application parameter when connecting, to enable usage stats.

Changed
- Remove requirement for running user to have CREATE SCHEMA privileges for --create-change-history-table when schema already exists (42)

Changed
- Bumped snowflake-connector-python to 2.4.2 and relaxed the pip dependency to ~= (pick the latest patch release 2.4.X on install)
- Bumped the docker container python version to 3.9 as snowflake-connector-python now supports this as of 2.4
- Created a 'Dockerfile-src' for users wanting to build schemachange themselves from source in Docker

Changed
- Renamed snowchange to schemachange
- Please be advised that the new default path for the change history table is now `METADATA.SCHEMACHANGE.CHANGE_HISTORY` (the schema name has been changed from `SNOWCHANGE` to `SCHEMACHANGE`). The easiest thing to do is rename the metadata schema in Snowflake. The other option is to leave it as is and use the `--change-history-table` (or `-c`) parameter to override the default.

Added
- Added a new optional parameter `--dry-run` to preview scripts that will be applied by snowchange.