Make the SDK ad hoc tests pass against the orchestrator (#2708)

## Motivation and Context
This PR refactors the client protocol test generator machinery to use a
client instead of calling `make_operation` directly, and then fixes the
ad hoc tests for the orchestrator.

The ad hoc tests revealed that overriding the signing region/service via
endpoint config was lost when porting SigV4 signing to the orchestrator,
so this PR updates the SigV4 `HttpRequestSigner` implementation to
restore this functionality. It is doing this in the signer directly
rather than via an interceptor since it should only run this logic when
SigV4 is the selected auth scheme.

Other notable changes:
- Adds `--no-fail-fast` arg to `cargoTest` targets so that all Rust
tests run in CI rather than stopping on the first failure
- Changes `EndpointResolver::resolve_and_apply_endpoint` to just
`resolve_endpoint` so that the orchestrator can place the endpoint
config into the request state, which is required for the signer to make
use of it
- Adds a `set_region` method to SDK service configs
- Deletes the API Gateway model and integration test from the SDK smoke
test since it is covered by the ad hoc tests
- Adds a comment explaining where the API Gateway model comes from in
the ad hoc tests
- Adds a `smithy.runtime.mode` Gradle property to `aws:sdk` and
`aws:sdk-adhoc-test` to trivially switch between middleware and
orchestrator when testing/generating locally

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