Charts generation improvments

[owl352]

Issue

After last update for charts, we have tiny bugs with they generation. One of this - last point may be not equal of end of time range
Also we need new param for generation with setted intervals count

Things done

Fixed all bugs with generation
Added new method for convertation timespan duration to ISO-8601 duration
Updated README.md
Updated all test for validators and transactions charts data

####################
  REQUEST EXAMPLE
####################

http://localhost:3005/validator/85F15A31D3838293A9C1D72A1A0FA21E66110CE20878BD4C1024C4AE1D5BE824/stats?start=2024-11-07T21:00:00.000Z&end=2024-11-11T20:59:59.999Z&intervalsCount=4

[
  {
    "timestamp": "2024-11-07T21:00:00.000Z",
    "data": {
      "blocksCount": 21
    }
  },
  {
    "timestamp": "2024-11-08T21:00:00.000Z",
    "data": {
      "blocksCount": 18
    }
  },
  {
    "timestamp": "2024-11-09T21:00:00.000Z",
    "data": {
      "blocksCount": 20
    }
  }
]

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