HTTPClient: Support concatenation of headers

[mattsches1]

Summary

Adds support for multiple response headers with identical keys; the main goal is to support setting several cookies requested by a server, which result in several "set-cookie" response headers.

Impact

If several headers with the same key are received in a response, they are concatenated using commas.

Related links

• Alignment with ESP8266 implementation (see esp8266/Arduino@1de0c34#diff-977435a9cc4619fa0b8b995085f6ae683485cf563722756bab57108b362da316)
• Fixes Multible http response headers with the same name get overwritten #4069

[CLAassistant]

All committers have signed the CLA.

[me-no-dev]

Could this be made to work with particular headers only? set-cookie in your case?

[mattsches1]

It could, but if I'm honest, I don't really see a point in it. And I thought aligning this with the ESP8266 implementation (from where I actually copied the code snippet) would be sensible,

But I was actually looking into a solution to handle cookies in the client and found the header concatenation would not really solve that (or require too much efforts around for my taste). In the meantime, I therefore implemented a cookie jar to enable the client to properly handle cookies. Just created a pull request for this: https://github.com/espressif/arduino-esp32/pull/6216

Feel free to discard any of these if they don't fit for whatever reason.

[me-no-dev]

Closing in favor of #6216