Add keychain synchronisation and authentication context

[crashdump]

Added the ability to pass an LAContext to the keychain key operations as well as the kSecAttrSynchronizable flag during the creation of keys.

[kornelski]

Please check the build failures on iOS.

It's unfortunate that GenerateKeyOptions has exhaustive public fields. This will need some workaround :/

[crashdump]

@kornelski, I fixed the MR & tests (I had forgotten to include a cfg) but I had not realised you had pushed your own changes and overwrote your push :( -- Would you mind re-applying?

Thoughts welcome on GenerateKeyOptions, it's indeed unfortunate!

[crashdump]

Thanks @kornelski, I can see the CI / Tests macos-latest tests are also falling on the main branch, so I'll ignore that for now.

As for the GenerateKeyOptions, we can either accept this as a breaking change (albeit not a major one since it's an Option which can be set to None by default) or introduce a new builder, similar to what was suggested in MR #220. I'll take your steer.

[kornelski]

I think the cleanest solution would be to hide the new field in GenerateKeyOptions behind a feature flag. Although theoretically flags are supposed to be purely additive, this breakage is small enough that won't be a problem in practice.

[crashdump]

Alright, it's done. Let me know your thoughts.

[crashdump]

I'm an idiot 🤦 - it's fixed now.