feat(transport): simplify NostrSigner initialization to accept hex string

[DevAnuragT]

Fixes #46

Description

This PR addresses the TODO in base-nostr-transport.ts by improving the ergonomics of the NostrSigner initialization. It allows the signer option to directly accept a hex-encoded private key string as a shorthand, automatically wrapping it in a PrivateKeySigner internally.

This is a non-breaking, additive change. Existing code that passes a completely instantiated NostrSigner will continue to function exactly as before.

Changes Made

• Updated BaseNostrTransportOptions.signer to accept NostrSigner | string.
• Added constructor logic to resolve the string into a PrivateKeySigner.
• Added 5 comprehensive unit tests targeting both client and server transports with string and NostrSigner inputs.

Testing Performed

• Ran type checks (tsc --noEmit) - passed.
• All new and existing unit tests pass cleanly.

[DevAnuragT]

Tests are passing now