refactor: migrate to smaller example for Snowflake

[farazdagi]

Description

• As agreed, replacing the huge Snowflake TUI app with a single file example, matching the other destinations
• Add cargo x seed: provisions a Postgres database with test tables and sample data (so that it is trivial to actually see examples in action)
• Add cargo x example <name>: runs any destination example, auto-injecting credentials from TESTS_* env vars
• Expand .env.example with setup instructions for all destinations (BigQuery, ClickHouse, Snowflake)

Usage

# One-time: seed a test database
cargo x seed

# Run any example 
source .env
cargo x example snowflake
cargo x example bigquery
cargo x example clickhouse

# Override defaults
cargo x example snowflake -- --db-name mydb --max-batch-fill-duration-ms 1000

# Re-seed with --force
cargo x seed --force --rows 5000

What was removed

The Snowflake TUI app (src/bin/snowflake/{main,tui,state,commands,logging}.rs) and snowflake_loadgen.rs have been extracted to a standalone repo. The simplified example follows the same pattern as all other destinations.

[coveralls]

coverage: 72.885% (-0.3%) from 73.187% — ref/snowflake-example into main