Implement index:stats

[Leodau]

What does this PR do?

This PR implements the recently added index:stats (@ Kuzzle Backend v2.10.0)
So it can be used directly, instead of having to use query to access it.

Todo:

• Implementation
• Unit Testing
• Doc + Code Snippet

How should this be manually tested?

1. Launch a Kuzzle stack.
2. Directly use sdk.index.stats()
3. Verify that you get stats that make sense.

Closes #608

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[codecov]

Codecov Report

Merging #613 (5f3684c) into 7-dev (fce607b) will decrease coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##            7-dev     #613      +/-   ##
==========================================
- Coverage   86.75%   86.73%   -0.02%     
==========================================
  Files          33       33              
  Lines        1502     1508       +6     
  Branches      262      262              
==========================================
+ Hits         1303     1308       +5     
- Misses        146      147       +1     
  Partials       53       53              

Impacted Files	Coverage Δ
src/controllers/Index.ts	75.00% <66.66%> (-1.20%)	⬇️
src/core/security/User.ts	100.00% <0.00%> (ø)
src/controllers/Security.js	95.00% <0.00%> (+0.07%)	⬆️

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[alexandrebouthinon]

To fix unit tests, you need to add this line at the top of src/controllers/Index.ts:

import { JSONObject } from '../types';