Support X25519 and X25519MLKEM768

[nisbet-hubbard]

Is your feature request related to a problem? Please describe.
Element X only supports NIST curves:

prime256v1:secp384r1:secp521r1->prime256v1

Describe the solution you'd like
Supporting X25519, which is better trusted.

Describe alternatives you've considered
X25519MLKEM768, the post-quantum alternative that has seen standardisation, should be the next move.

For context, see https://pq.cloudflareresearch.com/

Additional context
https://developer.apple.com/documentation/cryptokit/curve25519

[ara4n]

Element X already Curve25519 today, not NIST curves: https://spec.matrix.org/unstable/client-server-api/#molmv1curve25519-aes-sha2.

Meanwhile, PQXDH support is happening at matrix-org/vodozemac#120.

[nisbet-hubbard]

Element X already Curve25519 today, not NIST curves: https://spec.matrix.org/unstable/client-server-api/#molmv1curve25519-aes-sha2.

@ara4n Has this been implemented in the client? nginx shows that Element X only supports prime256v1:secp384r1:secp521r1.