Provide `From<ed25519::PublicKey> for x25519::PublicKey` instead of `TryFrom`

## Description

Is there a particular case where the conversion would fail? Particularly, the current implementation implies there might be a case where decompressing the ed25519 public key as a `CompressedEdwardsY` might fail. However, a comment also states that in the happy path of the conversion "`pk` is a valid `ed25519::PublicKey`". This is however already an invariant of the conversion (as we are converting from a `ed25519::PublicKey`), making one think the conversion is infallible in practice:

```rust
match curve25519_dalek::edwards::CompressedEdwardsY(y_bytes).decompress() {
Some(decompressed_edwards) => {
// `pk` is a valid `ed25519::PublicKey` hence contains valid `EdwardsPoint`.
// x25519 uses Montgomery form, and `x25519::PublicKey` is just a `MontgomeryPoint`.
// `MontgomeryPoint` can be constructed from `EdwardsPoint` with `to_montgomery()` method.
// Can't construct `x25519::PublicKey` directly from `MontgomeryPoint`,
// do it via intermediate bytes.
Ok(PublicKey::from_bytes(decompressed_edwards.to_montgomery().to_bytes()))
}
None => Err(crate::error::Error::ConvertError {
from: "ed25519 public key",
to: "x25519 public key",
}),
}
```

## Motivation

If the conversion is in fact infallible, the trait should be `From` instead of `TryFrom`. Currently https://github.com/iotaledger/streams relies on this conversion being infallible. If it in fact isn't, I'd appreciate some clarification so that we can adjust our error handling.

## Are you planning to do it yourself in a pull request?

Yes (upon confirmation)