Add custom metadata to uploaded files

[ronal2do]

Hi, not sure if is intended, but users are not able to save custom metadata to the vector when uploading a file from ragchat.

Usage example:

await ragChat.context.add({
  type: "pdf",
  fileSource: "./data/physics_basics.pdf",
  options: { 
    namespace: "user-123-documents",
    metadata: {
      theme: "physics_basics",
    }
  },
});

Given a setup RagChat instance,
When adding a new context from sources (e.g., PDF),
And including custom metadata,
Then users should be able to visualize their custom metadata in the Upstash vector console.

Please refer to the attached screenshot for a visual example of the issue:

Proposed solution on this PR extends config.options?.metadata for mapDocumentsIntoInsertPayload and turn it into a private method in order to be able to read the config.

[ronal2do]

In order to use data: expect.any(String), instead add the entiry data as string. same for ids, it will change on every run.

[ronal2do]

this was added to test a bad configuration, I know we must trust on TS, but on the file we have many anys already, but was used for the last 2 tests, it kinda was just to add more branch coverage

[ronal2do]

this was the biggest change here,

I'm destructuring the current metadata, to accept the provided metadata by the user, as we can do on raw texts

[CahidArda]

closing this PR as it was merged through #95. Thanks for the contribution!