[FEA]: Add support for custom metadata ingestion to nemo_retriever library

[randerzander]

Is this a new feature, an improvement, or a change to existing functionality?

Improvement

How would you describe the priority of this feature request

Currently preventing usage

Please provide a clear description of problem this feature solves

For Custom metadata feature, a user should be able to add key-value pairs for a specific source filename and apply filters on those key/vals during retrieval.

Describe the feature, and optionally a solution or implementation and any alternatives

Feature support should include the functional equivalent to this tutorial

Additional context

No response