Allow request header manipulation for HTTP functionality

[RhoSigma-QB64]

Is your feature request related to a problem? Please describe.
No, but it would make some things easier.

Describe the solution you'd like
When making HTTP calls to _OPENCLIENT it would be sometimes reqired to specify additional header fields or modify existing fields to request special behavior from the server. However, the current implementation does not allow for that.

Describe alternatives you've considered
The alternative would be to make a regular TCP/IP connection and make the GET request with your own custom headers yourself. However, that only works if the server supports http:// access. If only https:// is allowed, then it gets complicated, as the whole secure stuff had to be done manually somehow.

Additional context
Here's a simple description of what I would expect:
https://curl.se/libcurl/c/httpcustomheader.html

From the QB64 side the header fields could be provided in a string, each field separated by a LF or any other suitable control char. This string is then passed as new optional parameter to the _OPENCLIENT call, which would use it to modify the header as described in the document above before actually making the libcurl request.

[mkilgore]

Definitely agree this is a sorely missing feature, we also need things like the ability to set the verb and provide a request body. I think a good way to handle all this would be to have a kind of _HttpRequestNew that returns a handle you then pass to the _OpenClient() call. That gives us a lot of flexibility to add more stuff in the future without needing to keep adding _OpenClient() parameters. So just for headers it would be something like this:

r& = _HttpRequestNew

_HttpAddHeader r&, "Content-Type", "application/json"
_HttpAddHeader r&, "Authorization", "foo"

o& = _OpenClient("https://example.com", r&)

' On the output side we could add similar functions for reading response headers
c& = _HttpGetHeaderCount&(o&)
n$ = _HttpGetHeaderName$(o&, 0)
v$ = _HttpGetHeaderValue$(o&, 0)

Close o&

And then in the future we can introduce commands like _HttpAddRequestBody or _HttpSetVerb without any modifications to _OpenClient().

Whether we take the headers as a single string or individually is a separate question. I feel like it's probably easier to use if they can be provided individually but I'd need to try it out and see.

[RhoSigma-QB64]

That looks like a reasonable way to go.

[grymmjack]

PLEASE this would be awesome in this age of API.