Operation request: ECDSA key pair generation

## Summary
Operation to generate a ECDSA key pair

### Example Input
The desired curve: P-256, P-384 or P-521

### Example Output
private and public key as PEM (similar to how it is done for RSA)

Start by locating CyberChef's existing RSA key-generation operation and its associated operation registration and tests. Trace how the desired curve is accepted and how private and public PEM output is produced. Done means a new operation supports P-256, P-384, and P-521 and returns both keys in PEM format with coverage for those curves.

Written by the indexing model from the issue text.