add statistics endpoints

[vincentsarago]

closes #340

This PR adds 2 new endpoints per TilerFactories
GET - /statistics: get stats from a preview of a dataset
POST - /statistics: get stats from a feature covering a dataset

To Be Discussed

Per the HTML specification, list must be passed using multiple parameter (e.g p=1&p=2) and is also supported in openapi. When we started titiler we (I) didn't knew about this and used comma delimited string (e.g bidx=1,2,3) to allow list input. In the PR the statistics endpoints have 2 list inputs: c (categories) and p (percentiles). I've choose to keep the name small because those will be repeated in the query_param (e.g p=1&p=2 to define percentiles=[1, 2]).

I'm open to discuss either we should follow the spec or use comma delimited values. I've already raised this over #262 (comment)

IMO we should switch every list input to this, but as it's a breaking change I'll think starting to implement this for new QueryParameters is the safest way 🤷

Here is example of statistics result

$ curl http://127.0.0.1:8000/cog/statistics\?url\=https://rio-tiler-dev.s3.amazonaws.com/data/eu_webAligned_256px.tif&bidx=1
[
  {
    "min": 38,
    "max": 255,
    "mean": 125.75282900811122,
    "count": 262106,
    "sum": 32960571,
    "std": 36.320596765212024,
    "median": 102,
    "percentile_2": 0,
    "percentile_98": 178,
    "valid_percent": 25
  }
]

$ curl http://127.0.0.1:8000/cog/statistics\?url\=https://rio-tiler-dev.s3.amazonaws.com/data/eu_webAligned_256px.tif&bidx=1&categorical=true&c=1&c=2&c=3
[
  {
    "categories": {
      "1": 0,
      "2": 0,
      "3": 0
    },
    "valid_percent": 25
  }
]