feat: add embedding implementation for openai provider

[clinia-brando]

Summary

• Adds openai embedder implemention
• Changes the pattern of the openai provider to match the one of vercel/aisdk e.g.

	// Initialize the OpenAI provider
	provider := openai.NewProvider()

	// Create a model
	model := provider.NewLanguageModel("gpt-4o-mini")

• Refactor openai LLM to use provider config; default model init updated.

How was it tested?

Added unit tests and examples

Community Contribution License

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