feat: Add X25519 key derivation functions

[FrederikBolding]

Explanation

Add functions for deriving X25519 public keys and shared secrets.

React Native benchmark:

Finished x25519 public key benchmark: WebCrypto = 3 ms JS = 78 ms
Finished x25519 shared secret benchmark: WebCrypto = 3 ms JS = 90 ms

References

https://consensyssoftware.atlassian.net/browse/WPC-1336

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

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Note

Medium Risk
Introduces new cryptographic key-agreement APIs where misuse could affect secret derivation, though behavior is delegated to WebCrypto with strict length checks and RFC test vectors.

Overview
Adds X25519 key agreement to @metamask/cryptography as a dedicated subpath export (@metamask/cryptography/x25519), exposing getPublicKey and getSharedSecret for 32-byte keys.

Both operations share a WebCrypto deriveBits path: raw private keys are wrapped in a fixed PKCS#8 header before import, public keys use raw import, and scalar multiplication covers public-key derivation from the RFC 7748 base point and ECDH-style shared secrets. A small toUint8Array helper normalizes BufferSource inputs (including ArrayBuffer and DataView).

The changelog documents the new API, and tests cover custom vectors plus RFC 7748 Section 6.1 Alice/Bob cases, input-type variants, and explicit errors for wrong key lengths.

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[Mrtenz]

While the underlying functionality is correct here, using getSharedSecret to get a public key is a bit confusing. Maybe we should extract the functionality into a new function (scalarMultiply or something), and call that from getPublicKey and getSharedSecret.

async function scalarMultiply(privateKey: BufferSource, publicKey: BufferSource): Promise<Uint8Array> {
  // Current implementation of `getSharedSecret`.
}

export async function getPublicKey(privateKey: BufferSource): Promise<Uint8Array> {
  return await scalarMultiply(privateKey, X25519_BASE_POINT);
}

export async function getSharedSecret(privateKey: BufferSource, publicKey: BufferSource): Promise<Uint8Array> {
  return await scalarMultiply(privateKey, publicKey);
}

[FrederikBolding]

Yeah, that's a good idea. It's a bit of a hack, so good to make it clearer.

[FrederikBolding]

935bff1