Encapsulate encoding and decoding of HTTP header fields

[mikee47]

• Move HttpHeaders from structures.h into a separate module, and expand
• Replace common HTTP header field names with enumerated type - eliminates errors due to mis-typing, simplifies code
• Replace instances of NULL with nullptr as appropriate in any updated files
• Wrap constant string data with _F() or F() as appropriate

Methods no longer used:

• HttpResponse::hasHeader()
• HttpRequest::getHeader()

[mikee47]

@slaff I've held-off making any changes to Samples (HttpClient, HttpClient_Instapush, HttpServer_ConfigNetwork, MeteoControl, but can add those if you want.

[mikee47]

I originally implemented the HttpHeaders class as a HashMap(enum, String) but because that wouldn't support custom header fields directly I elected to keep it as a HashMap(String, String) for simplicity. Clearly using the enum for lookups is a lot faster so I may revisit this at some point.

[slaff]

Clearly using the enum for lookups is a lot faster so I may revisit this at some point.

We should not forbid or make it very complicated to set custom headers. If there is an elegant way to use enum as lookup and also allow custom headers I will be keen to see it (Can be done also as a separate PR ).

[slaff]

HttpClient, HttpClient_Instapush, HttpServer_ConfigNetwork, MeteoControl, but can add those if you want.

As long as the changes are related to the main goal of that PR then go ahead. Because the samples are the first thing that new developers copy and modify we should give them a very good start by applying the best practises in using Sming and C++.