[Feature] Add Modelsell as an OpenAI-compatible provider

[modelsell]

What Would You Like to See with the Gateway?

Please add Modelsell as a named provider in Portkey Gateway.

Modelsell exposes an OpenAI-compatible API:

• API base: https://modelsell.com/v1
• Models: GET /v1/models
• Chat Completions: POST /v1/chat/completions
• API keys: https://modelsell.com/console/token
• API documentation: https://modelsell.com/docs/api-reference

The integration should use bearer-token authentication and discover model IDs dynamically from /v1/models rather than hardcoding a catalog.

Context for your Request

Portkey already supports OpenAI-compatible aggregation providers. A named Modelsell integration would let users select it through the existing provider routing/configuration path while preserving the standard OpenAI request and response format.

I can contribute the implementation and focused tests if this direction is acceptable.

Disclosure: I represent Modelsell.

No live authenticated Modelsell API call was performed.