Update Rust example to include non-admin logic and align with docs

[danielfrankcom]

This PR updates the Rust example.

It includes the following changes:

• Synchronize the example with the publicly available docs here
• Synchronize the example with other approved examples
  • Remove UUID generation
  • Use fixed telephone
  • Align create table DDL
• Clean up and improve the documentation/comments
• Minimize Cargo dependencies
• Add non-admin connection to existing example
• Integrate token refresh logic from rust/sqlx: Refresh the token every 10 minutes #124
• Allow configuration with environment variables

I manually tested the token refresh logic by shortening the expiry duration, and changing the pool to lazily create connections. If I wait longer than the initial token expiry, then the pool can still connect lazily, indicating the token was refreshed.

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