Add support for DreamGen API.

API docs: https://dreamgen.com/docs/models/opus/v1
API keys: https://dreamgen.com/account/api-keys

I decided to base this on the text-completion API since it's more
flexible with SillyTavern's prompt formating capabilities.

This also means that custom context and instruct settings are required.

Will add documentation in a followup PR.

src/endpoints/secrets.js

@@ -31,6 +31,7 @@ const SECRET_KEYS = {
     CUSTOM: 'api_key_custom',
     OOBA: 'api_key_ooba',
     INFERMATICAI: 'api_key_infermaticai',
+    DREAMGEN: 'api_key_dreamgen',
 };
 
 // These are the keys that are safe to expose, even if allowKeysExposure is false