Add example code to use Snowflake based bridges in arti-client

Problem to solve

Currently, bridge docs are limited to one example using obfs4, whereas there are also Snowflake-based bridges which could be used by a prospective developer in their program.

Not having a Snowflake example just adds more friction for their use, especially in a brand-new API which doesn't have much outside documentation or usage.

Further details

Having a basic config written in the example code given for BridgesConfig using the default torproject.org broker for Snowflake proxies would go a long way towards boosting adoption of Snowflake.

Proposal

Add one more bridge in the example code which instead makes the connection via Snowflake. The rest of the code would be mostly unchanged.