Allow for specified Cargo commands to be run on a subset of the integration tests (#1165)

This commit allows for the `modules` and `cargoCommands` properties to
be specified when running the `codegen-test` and `codegen-server-test`
Gradle modules. The `modules` property allows one to only generate a
subset of the integration test services, while the `cargoCommands`
property allows one to specify the Cargo commands to be run on the
generated Rust crates.

This functionality can be useful to reduce development iteration cycles. For
instance, to only run `cargo test` on the integration test
`simple.smithy`:

```sh
./gradlew codegen-test:build -P cargoCommands='test' -P modules='simple'
```

This commit also refactors the buildscripts of the `codegen-test` and
the `codegen-server-test` modules, extracting shared functionality to
`buildSrc/src/main/kotlin/CodegenTestCommon.kt`.