fix(s2n-quic-rustls, s2n-quic-tls): enable building with default features

[jmayclin]

Description of changes:

• add alloc dependency to rustls and tls crates
• add default dependency to h3 crate
• add CI step to check that each crate builds.

Note that the name of this PR is confusing. "default" features is literally referring to "the set of features included when build is invoked directly on the package", and not necessarily cargo's concept of "default-features".

Currently we are unable to release to crates.io because s2n-quic-rustls and s2n-quic-tls do not build with default features. This happened because in a #1633 , components that s2n-quic-rustls depends on were moved behind the alloc feature flag.

We failed to detect this in our CI because the workspace build builds from s2n-quic with default features enabled. This build happens successfully because the alloc feature is included in default.

An issue that seems to describe in a bit more detail is linked here: rust-lang/cargo#8366

Call-outs:

I'm still a bit uncertain on our overall story for no-std usage. It might be good to add an example showing an intended use case.

Why not just dry run publishing/packaging?

Because that relies on artifacts being published to crates.io

For example consider that we just version bumped things so that for crate A which consumes B, we now have
A = 1.0.0 depends on B = 1.0.0.

• dry run publish B = 1.0.0 succeeds
• dry run publish A = 1.0.0 fails because B 1.0.0 can't be found on crates (because it wasn't actually published)
  The same problem applies to the cargo package workflow.

Testing:

Tested by invoking the individual package builds. Also adding a step for that to the CI. The list of packages that are will be dynamically updated.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[toidiu]

We seem to already have a msrv that we parse from the rust-toolchain below in env. Do we also need this?

[jmayclin]

Good call out, will update.

[toidiu]

Why not just dry run publishing/packaging?

This also fails on crates that are marked publish = false

[jmayclin]

I opened a draft PR with just the workflow changes to make sure that it breaks on our current state, and the task was failed successfully #1676. The new workflow failed on h3, tls, and rustls as expected.