HTTPRequest: Expose and add methods to make working with headers easier

[Kubulambula]

Closes 4734

Exposes has_header() and get_header_value() methods and adds get_dictionary_from_headers method.

Also converts headers to lower-case in HTTPClient::_get_response_headers_as_dictionary() as well as in HTTPRequest::get_dictionary_from_headers() to keep consistensy.

This should be a done right version of 62962

[timothyqiu]

Since the headers are returned as a dictionary, I think it'll be easier to work with if the keys are always lower-cased.

Users can then use lower-cased names to get the header value. Otherwise, they have to iterate through the keys and make case-insensitive compares.

[Kubulambula]

Since the headers are returned as a dictionary, I think it'll be easier to work with if the keys are always lower-cased.

Users can then use lower-cased names to get the header value. Otherwise, they have to iterate through the keys and make case-insensitive compares.

I have done it this way in order to keep consistency with HTTPClient.
It would make sense to make the keys case-insensitive in my opinion, but do it the same way in both cases

[Kubulambula]

Should be all good now

[mhilbrunner]

The commits need to be squashed into 1 commit :)

[pafuent]

@Kubulambula if it's OK with you, I can create a new PR with this changes and add you as co-author.

[Kubulambula]

@Kubulambula if it's OK with you, I can create a new PR with this changes and add you as co-author.

No problem at all. I would be happy if this functionality makes it in!

[akien-mga]

Superseded by #97174. Thanks for the contribution!