RFC30: Add test that runs with unstable flag on

[thomas-k-cameron]

Motivation and Context

This is a sub-PR of #2615.

Changes

It adds test-with-serde check in github actions, which runs all the tests with RUSTFLAG="--cfg aws_sdk_unstable" env variable.

We need this to run tests with unstable feature enabled.

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[rcoh]

I think something like this is necessary. We probably want a single parallel task to sanity check everything with the flag enabled but do most testing with it disabled. Need to think about this. The main issue is the coverage that we're losing with the flag disabled (which is, of course, where most customers are going to be)

Maybe one CI target test-cfg-serde that runs some targeted tests of the serde behavior?

[thomas-k-cameron]

@rcoh

I added test-with-serde to ci.yml.
https://github.com/smithy-lang/smithy-rs/pull/2851/files

I think it's working.
https://github.com/smithy-lang/smithy-rs/actions/runs/7279552536?pr=2851

[thomas-k-cameron]

I got 2 errors.

1. :jar: No valid plugin descriptors were found in META-INF/gradle-plugins from generate

and

2. Verify client TLS configuration

output: Connecting to https://localhost:40835 with root CA bundle from /tmp/tmpybhd04f_/2: Adding root CA: O=Fake Certificate Authority (65:84:52:d7:3e:03:25:d5)DispatchFailure { source: ConnectorError { kind: Io, source: hyper::Error(Connect, Custom { kind: Other, error: Custom { kind: InvalidData, error: InvalidCertificate(NotValidForName) } }), connection: Unknown } }

Since I made no changes to both jobs, I have no idea where it's coming from :(
I don't think my changes are causing them.

[thomas-k-cameron]

I think this one's ready for review.