Hugo: Allow user-defined HTTP headers with getJSON and getCSV

[anthonyfok]

As suggested in #3395 "Add an HTTP Accept header when downloading JSON", it would be nice if one could add user-defined HTTP headers with getJSON and getCSV, especially the case when some kind of token is needed for authorization, e.g., as described on https://docs.tcgplayer.com/docs/getting-started where a Authorization: bearer BEARER_TOKEN HTTP header is required for access.

[roya3000]

To have this feature would be very nice. Possible implementation

func (ns *Namespace) GetJSON(headers string, urlParts ...string) (interface{}, error)

Usage in template

{{ $json := getJSON "headerKey:value, header2Key:value2 "url" }}

Example with Authorization:Bearer

{{ $header := "Authorization:Bearer " }}
{{ $token := .Site.Params.API_TOKEN }}
{{ $authHeader := printf "%s%s" $header $token }}

{{ $json := getJSON $authHeader "url"}}

I can implement this feature based on the provided function interface.

Edit:
Probably changes in GetJSON is not a good idea and would be better to have a separate function.

[moorereason]

@roya3000,
I see your edit, but I want to make it clear to anyone reading this issue that your proposal would break the existing API by making the headers required for every invocation (ie. {{ getJSON "" $url }}). That's not going to work.

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