Add 'mode' featurecount to vector REST API

[wlorenzetti]

Add new mode featurecount to vector REST API /vector/api.

Give a QGIS layer id a project id, this 'mode' returns a dict in which for each rule-key of the layer style categories the number of features identified by that style category is reported.
I.e.

For a API call like

GET /vector/api/featurecount/qdjango/321/countries_simpl_adf0132d_df8d_4fa3_a55b_65b00f790256/

it returns:

{
    "result": true,
    "data": {
        "0": 688,
        "1": 296,
        "2": 524,
        "3": 613,
        "4": 336
    },
    ...
}

style GET/POST query parameter is optional, if not sent is used layer current style.

Closes: #

[wlorenzetti]

@volterra79 said me when can I merge it.

[volterra79]

@volterra79 said me when can I merge it.

@wlorenzetti ok for me