Rust client quickstart updated for 0.12

[jdetter]

Overview

• The purpose of this PR is mainly to get the rust client quickstart up to speed with the new API, we will do another pass on this quickstart for 1.0.

Rendered: https://github.com/clockworklabs/spacetime-docs/blob/200660cbc080e5fba2bb3792e7285f0a0d31f066/docs/sdks/rust/quickstart.md