Expose proxy_user and proxy_password from SnowflakeConnection to SnowflakeCredentials class

[brifitz]

resolves #
docs dbt-labs/docs.getdbt.com/#

Problem

Unable to connect to Snowflake in a corporate environment as proxy_user and proxy password are not attributes of SnowflakeCredentials. These attributes are attributes of the underlying SnowflakeConnection, just like proxy_host and proxy_port, however the SnowflakeCredentials class does not currently expose them.

Solution

Successfully connected to Snowflake after hard-coding the proxy_user and proxy password in the snowflake.connector.connect(...) in the SnowflakeConnectionManager class.

This PR exposes the proxy_user and proxy password attributes of the underlying SnowflakeConnection class to the SnowflakeCredentials class so that the above workaround is not needed.

Checklist

• [ x] I have read the contributing guide and understand what's expected of me
• [ x] I have run this code in development and it appears to resolve the stated issue
• [ x] This PR includes tests, or tests are not required/relevant for this PR
• [ x] This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX