P256 keys using Secure Enclave and Android StrongBox

[berendsliedrecht]

This PR contains the functionality to add support for hardware-backed P256 keys on iOS and Android Using iOS Secure Enclave and Android StrongBox. This is done by using the secure-env library. iOS is accessed via Security.framework and Android via JNI. Using these features you must build the library with mobile_secure_element or askar-crypto directly with p256_hardware.

During the implementation I ran into some issues due to askar-crypto being no_std. because of this I made the rng: impl KeyMaterial optional as well as the id: &str. My initial intention was to create a structure or enum for KeyGenOps, but due to not always having access to an allocator I could not leverage Box and dynamic dispatch. If there are any solutions for this, I would be happy to change the current implementation.

askar-crypto now also has a method to get the key by id. Which ties askar-crypto directly to storage, but I could not find any other way of doing this due to the nature of Secure Elements.

~~Over FFI, when supplying an id to askar_key_generate, the key will be created on hardware. This seems a bit too implicit for me. My next approach would be generate a random id, uuid or something and simply pass in a boolean for hardware_backed.~~

Implemented that we generate an id now for secure element keys and the user just has to provide hardware_backed argument over FFI as an i8.

Still in draft for now, as I have to do some testing.

Will also include an updated python wrapper (only software as key_backend allowed) and an updated node.js wrapper. React Native will be done in #246 as that will also include the React Native Example app and move to pnpm.