fix JWK endpoints returning empty keys

[olamilekan000]

What kind of change does this PR introduce?

Bug fix

What is the current behavior?

The JWKS endpoint (/auth/v1/.well-known/jwks.json) returns an empty keys array ({"keys": []}) for local Supabase instances because signing keys are not automatically generated during supabase init. Users must manually generate signing keys using supabase gen signingkeys and configure signing_keys_path in config.toml.
Issue: #4614

What is the new behavior?

• Automatically generates a default RS256 signing key during supabase init
• Saves the key to supabase/signing_keys.json (already in .gitignore)
• Skips generation if the signing key file already exists
• The JWKS endpoint now returns the public key, matching cloud behavior

Additional context

Add any other context or screenshots.

[olamilekan000]

@sweatybridge kindly review. Thanks