Allow to set OPENAI_API_BASE_URL manually

[zadjadr]

My usecase:

Create an external Secret via extraResources and use it to populate OPENAI_API_BASE_URL

[0xThresh]

Thanks @zadjadr, I started testing this but ran into an edge case templating the chart when setting openaiBaseApiUrl and pipelines.enabled set to true. I'll keep playing with this over the weekend and do a full test before merging this in, but thank you for the great work getting this started!