fix(open-webui): wrong behavior on openai api keys

[this-is-tobi]

Fix #310

This pull request updates the Open WebUI Helm chart to version 8.12.3 and refines the configuration for handling OpenAI API keys and endpoints. The changes clarify and improve how multiple OpenAI API keys are provided and used, especially in deployments with Pipelines enabled, and ensure that environment variables are only set when relevant values are present.

Chart version and documentation updates:

• Bumped chart version from 8.12.2 to 8.12.3 in Chart.yaml and updated the version badge in README.md to reflect the new release. [1] [2]

OpenAI API keys configuration improvements:

• Changed the default value of openaiApiKeys in values.yaml from a preset list to an empty list, and updated documentation to clarify how keys should be provided, especially when Pipelines are enabled (the list should match the number of URLs, plus one if Pipelines are used). [1] [2]
• Updated the Helm template logic in workload-manager.yaml to only set the OPENAI_API_KEY and OPENAI_API_KEYS environment variables if the corresponding values are provided, preventing unnecessary or empty environment variables from being set.

[this-is-tobi]

@westbrook-ai the default openaiApiKey: "0p3n-w3bu!" should now be injected in env only if pipelines.enabled: true and openaiBaseApiUrl: "" so it can now be override using envFrom.