Sap generative ai hub

[mbrner]

This PR is to add the SAP Generative AI Hub as a model provider (reopened from #728).

The SAP Generative AI Hub provides a proxy for commercial models such as OpenAI and the ability to provide your own models. OAuth is always used for authentication, i.e. if you use OpenAI models via the Generative AI Hub, you do not need an API key, but the OAuth token.

In this PR, I have slightly modified the OpenAI class and introduced protected methods for determining the URLs and for generating the request headers. Based on the modified class, I added a SAPGenAI class that adapts the two methods for the Gen AI Hub.

ToDos:

• Check how to adjust config validation, because the GenAI Hub's azure openai/apiType does not require engine + apiKeyand the openai type does not require apiKey.

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[sestinj]

@mbrner This looks really solid. Can we group together those options into a single interface like this?

interface SapGenAiOptions {
    // SAP Gen AI Core options
    resourceGroup?: string;
    authURL?: string;
    clientID?: string;
    clientSecret?: string;
}

Will help avoid needing to repeat all of these variable names repeatedly. We didn't do this with the Azure OpenAI options but I may go back and do that on my own—it seems like things will start to clutter without this.

Otherwise I'd consider this basically ready as long as it seems to be working on your side

[sestinj]

There's a set of clauses for the models array inside of an allOf that has good example of how to conditionally require certain properties. In this case after grouping into the interface, you can just require the one.

A sample: https://github.com/continuedev/continue/blob/a3c6d04e6dfa88fa64a0a1b738509d930a9147f3/extensions/vscode/config_schema.json#L276C9-L288C11

[sestinj]

Hi @mbrner, wanted to check in on this—happy to make these changes myself if it's easier, just let me know! Otherwise, since the PR is in draft state, I'll wait until I get a green-light from you.