Extension of layerSpecificExport to allow more config options

[MattiasSp]

Description
The layerSpecificExport options for the infoWindow are great, but could be more flexible with the request specifications.

Adding the option to include URL-parameters and choose between making a POST- or a GET request would go a long way to making this feature more general and make it possible to export selected features to many different types of APIs.

Proposed solution
Adding a bunch of config parameters to the layerSpecificExport which control how the requests are made.

Alternatives considered
Adding more hardcoded export modes other than simpleExport and layerSpecificExport would make the configuration simpler as each mode could be better tailored to a specific type of API. But I prefer the proposed solution because hard coded modes would make the code more bloated and repeated, and would also require the code to be updated to match any changes to the APIs.

[steff-o]

Will the bunch of parameters have some sort of variable substitution like:

{
    "url": "https://reportserver.kommun.se/reports",
    "bunchOfParmeters" : {
        "PageSize": "{{localStorage.userSettingPageSize}}",
        "Filename": "{{feature[0].Attribute['name']}}",
       "format" : "pdf"
    }
}

... or is it just constants?

[MattiasSp]

Will the bunch of parameters have some sort of variable substitution like [...] or is it just constants?

The posted PR only handles features from selected attributes and the special case of "valueless" parameters. But I like your idea of allowing generic code like localStorage.userSettingPageSize to be substituted as well!

That, authentication and posting other data formats would be awesome further additions :)