Support for Ed25519 → X25519 Key Conversion

### New API Proposal: Ed25519 → X25519 Key Conversion

#### Motivation:

This conversion is necessary for [the X3DH Key Agreement Protocol](https://signal.org/docs/specifications/x3dh/), which is used to establish end-to-end encrypted sessions using [the Double Ratchet algorithm](https://signal.org/docs/specifications/doubleratchet/).

#### Importance:

The current package does not appear to expose functionality for performing this conversion.
I was forced to use [libsodium](https://doc.libsodium.org/advanced/ed25519-curve25519):

```c
crypto_sign_ed25519_pk_to_curve25519(x25519_pk, ed25519_pk);
crypto_sign_ed25519_sk_to_curve25519(x25519_sk, ed25519_skpk);
```

#### Proposed API:

```swift
Curve25519.KeyAgreement.PrivateKey {

init(unsafeFromSigningPrivateKey key: Curve25519.Signing.PrivateKey) {...}
}

Curve25519.KeyAgreement.PublicKey {

init(unsafeFromSigningPublicKey key: Curve25519.Signing.PublicKey) {...}
}
```

Start by reviewing the proposed Curve25519.KeyAgreement.PrivateKey and PublicKey initializers and their Curve25519.Signing key inputs. Compare the requested Ed25519-to-X25519 behavior with the package's existing API, then define tests that confirm both conversions before considering the feature complete.

Written by the indexing model from the issue text.