Allow retrieving vectors when fetching documents

[xarantolus]

Pull Request

This adds fields that are required for retrieving vectors from Meili.

Related issue

I didn't open an issue as I just fixed it - however, if you want a more proper discussion about how to add this, I can open one.

What does this PR do?

• Adds the retrieveVectors option for document fetching endpoints to the Go client
• This is required for end-users who work with the vectors stored in their index

PR checklist

Please check if your PR fulfills the following requirements:

• Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
• Have you read the contributing guidelines?
• Have you made sure that the title is accurate and descriptive of the changes?

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 86.05%. Comparing base (40ea18f) to head (d262492).

Files with missing lines	Patch %	Lines
index_document.go	0.00%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #607      +/-   ##
==========================================
- Coverage   86.23%   86.05%   -0.19%     
==========================================
  Files          14       14              
  Lines        2848     2854       +6     
==========================================
  Hits         2456     2456              
- Misses        282      287       +5     
- Partials      110      111       +1     

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[Ja7ad]

@xarantolus Possible write unit test for this param?