extras: Provide initializers for RSA keys from RSA parameters

[simonjbeaumont]

Motivation

We currently offer APIs to construct RSA keys from PEM and DER representations but we have no way of constructing they key from its constituent RSA parameters: n and e for public keys, and n, e, d, p, and q for private keys. Sometimes these are what you have to hand, e.g. in a JSON Web Key.

Modifications

Provide initializers for RSA keys from RSA parameters for all the RSA key types.

To implement this, we leverage the fact that we are making use of BoringSSL in _CryptoExtras on all platforms. We need this because there are no APIs to construct the underlying key type on Darwin platforms from these parameters. So we do this by first creating a BoringSSL key, serializing it to PEM format, and then constructing a platform-specific key from the PEM representation.

Result

New APIs to construct RSA keys from RSA parameters.

[simonjbeaumont]

Thanks for the feedback @Lukasa. I think this is ready for another pass 🙏

[Lukasa]

Very nice, just a little note.

[0xTim]

cc @ptoffy since we've implemented that ourselves in JWTKit - we should either wait for this or ensure our APIs are internal so we can switch over without an API break

[ptoffy]

@0xTim we have our own key wrapping SwiftCrypto's so we should be fine. We can switch to this without breaking anything

[Lukasa]

Great, looks good to me, minus the inlinable thing.

[Lukasa]

Generalised nit: @inlinable on generic methods, as far as possible. This feedback applies to any new generic function or type.

[simonjbeaumont]

FWICT there is a very deliberate use of private for the backing key types and fileprivate for the type aliases which is why existing generic initialisers are not currently inlinable either (e.g. the PEM and DER initialisers).

I think we'd probably need to take a wider view to achieve this so should we defer this to a future PR?

[Lukasa]

Sounds good.

[simonjbeaumont]

@swift-server-bot test this please