Add an example of using a pooled bb8 connection with rustls

[ThouCheese]

No description provided.

[weiznich]

Thanks for adding this example 👍

Beside the minor change to the example I would like integrate the example into repository. That means:

• Add it to the workspace here: https://github.com/weiznich/diesel_async/blob/8d0857503d568d238b9e73d2a22bb652c08e796c/Cargo.toml (requires introducing a workspace item there first)
• Add it to the CI setup here:

  diesel_async/.github/workflows/ci.yml

  Line 117 in 8d08575