feat: add size to index-stats API response

[hmacr]

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  feature

• What is the current behavior? (You can also link to an open issue here)
  Add Index Size (GB) in stats[ENHANCEMENT] #374

• What is the new behavior (if this is a feature change)?
  Index size will be returned as a field named size in the index-stats API response

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  No

• Have unit tests been run against this PR? (Has there also been any additional testing?)
  Yes

• Related Python client changes (link commit/PR here)
  There is no index/stats client method exposed in py-marqo. Do you want it to be added as part of this change?

• Related documentation changes (link commit/PR here)

• Other information:

• Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes/features)
• Docs have been added / updated (for bug fixes / features)

[pandu-k]

Hi @hmacr ! Thanks for the PR!

1. Is it possible to rework this using the OpenSearch stats endpoint?

GET /{index}/_stats

indices.{index}.total.store.size_in_bytes

2. Can you add some a unit test for this please?
   See here for the existing get_stats tests
   https://github.com/marqo-ai/marqo/blob/mainline/tests/tensor_search/test_get_stats.py

3. Can you run please run tests.
   Read here for how to do that: https://github.com/marqo-ai/marqo/blob/d39f2312e9e289b0c54f49e14cddd49095808ad4/CONTRIBUTING.md#unit-tests
   You can run just the tensor_search tests, which are faster and use less resources, with the following:
   tox -- tests/tensor_search
   If you are having a lot of trouble running tests, I can create branch in our repo from your fork and run our pipelines against it, and pass you the feedback

[hmacr]

@pandu-k Apologies, I understand it's been a long time since I created the pull request; I'm trying to be more active on open source these days.

About the review comments, I've addressed them and ran the unit-tests as well (attaching the screenshots; there was one failure due to an unavailable remote repo). Please let me know your feedback.

[hmacr]

I saw that the computed size of the documents uploaded in this test was changing on each run; so figured len check would be a good assertion here, and added unit-tests for the bytes to human readable format function here