Add feature flag for enabling FIPS.

[tobz]

As described.

Adds a new feature flag, fips, to enable FIPS in rustls.

Verified that when enabling the new fips feature flag, that FIPS mode is used in aws-lc-rs (by seeing aws-lc-fips-sys being used.).

Resolves #267

[cpu]

Thanks!

[djc]

So the fips flag only works on Linux -- this will need some adjustments in the CI workflow to make sure we don't run --all-features on other platforms.

[tobz]

I can try and address that here, as well. Probably also worth adding a note to the documentation that it is only supported on Linux, as well.

[tobz]

Alright, pushed an update to the README. Let me know what you think about that.

Also pushed an update to the CI to try and automatically collect the set of features to test. I did this for all-features-but-FIPS and for the "defaults + ring" features. It's definitely a little more complex, but it seems, perhaps, somewhat better from an automation standpoint so that updates to the default features don't get stale in the CI definitions.

Totally understand if it feels too complex, though: I'm happy to switch it to just be hard-coded.

If you think it's OK, then I think you just need to approve the workflow to run.

[cpu]

Totally understand if it feels too complex, though: I'm happy to switch it to just be hard-coded.

I suspect that will be our preference here based on how we've handled this in other Rustls org repos. @djc Do you concur?

[tobz]

I'll just approach it that way then.

Also means less faffing about debugging the workflow since it means you having to approve them to run every single time I make a change. 😅

[djc]

Yeah, using yq in the workflow definition is a little too meta for me, and relying on tools that aren't all that familiar to the maintainers (at least to me).

[tobz]

Alright, this should be all set now.

[cpu]

LGTM. Thank you. The README update is a nice touch 👍

[djc]

Thanks! (Bypassed the merge queue in order to squash.)