Fix adhoc SDK codegen and refactor SDK CI (#1624)

* Fix `aws:sdk-codegen-test` build
* Add config for SDK endpoint/readme generation
* Rename `sdk-codegen-test` to `sdk-adhoc-test` and update README
* Add `sdk-adhoc-tests` to CI
* Refactor CI scripts

The `check-rust-runtime-and-tools` script was getting beyond 20
minutes to run, so this commit splits it into `check-aws-config`,
`check-rust-runtimes`, and `check-tools`.

This also enables most of the checks to run before the smoke test SDK is
generated, which should deliver more immediate feedback on PRs.

* Give the canary its own CI step