ml-kem: make `encapsulation_key()` public

[commial]

Hello,

For easier integration with others crates and simplify some ml-kem use cases, it would be nice to have a way to obtain the encapsulation key from the decapsulation one.
It seems that this is already possible and done inside the crate, but the related function is not published publicly:

KEMs/ml-kem/src/kem.rs

Line 109 in a8315ad

pub(crate) fn encapsulation_key(&self) -> &EncapsulationKey<P> {

Would it be possible to make it widely available, or am I missing an already existing method to obtain the encapsulating key from the decapsulation key?

[tarcieri]

FWIW we had some discussion of traits for this here: RustCrypto/traits#1559 (comment)

cc @rozbb @incertia

[tarcieri]

Barring some resolution on what the traits should look like, I think we can start by making encapsulation_key public

cc @bifurcation

[incertia]

I don't see anything wrong with making it public, although I'm not the original owner of this code.

[tarcieri]

If someone wants to open a PR we can move forward on making it public