Send Direct Message through specified Relays

[nobu-maeda]

Describe the enhancement
Create a functional interface to make it easier to send Direct Messages only through specified relays

Use case
This is in compliance to NIP-65, where it says and I quote

DMs SHOULD only be broadcasted to the author's WRITE relays and to the receiver's READ relays to keep maximum privacy.

Additional context
One way to do this is to create flexibility for the application layer to determine which relays to send the DM to instead of making the nostr client library implement and adhere to the guideline of NIP-65. The other way is to make the library have this behavior by default.

Either way, a dedicated functional interface to call into to allow sending of direct messages only to specified Relays is desirable.