[Format v2] Cryptography specifications

[extiop]

Once the format v2 is finalized, a CRYPTO.md must be added to detail cryptography specifications.

It can be relevant to talk about cryptography agility by stating that at the moment only one cryptography suite is available. Thus, MLA lacks crypto agility which is an encouraged property regarding PQC by ANSSI. However, HPKE improves this aspect of MLA (cf. https://github.com/ANSSI-FR/MLA/issues/211).