feat: implement managed identity (closes #56)

[sinedied]

Purpose

• Add full managed identity support
• Switch the vector DB to Azure AI Search (as Azure CosmosDB for MongoDB vCore currently doesn't support Managed Identity)

Previous code using API keys and Azure CosmosDB for MongoDB vCore has been branched here: https://github.com/Azure-Samples/serverless-chat-langchainjs/tree/mongodb-vcore

Does this introduce a breaking change?

[x] Yes
[ ] No

Pull Request Type

What kind of change does this Pull Request introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Documentation content changes
[ ] Other... Please describe:

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[glaucia86]

LGTM