vectorstores: add WithEndpoint option for Azure AI Search

[thecoons]

Description

Currently, the only way to configure the target endpoint for the Azure AI Search vector store is through the environment variable AZURE_AI_SEARCH_ENDPOINT.
This PR adds the ability to configure the endpoint during store construction, consistent with other configuration options.

Existing implementation

https://github.com/tmc/langchaingo/blob/main/vectorstores/chroma/options.go#L35

PR Checklist

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[Jiehong]

As discussed in our chat, this change LGTM.

@tmc Hi, is this PR acceptable? We're starting to use this lib, and we are realising the Azure part is not as easy to use as we thought.

We would like to make more PRs to improve that. Would that be ok?

[tmc]

Thanks for your contribution, LGTM.