[ENH] Allow custom headers to configure for feed

[gjimish]

Is your feature request related to a problem? Please describe.

Allow to pass other headers as well for feed url.

Describe the solution you'd like

simplepie/src/SimplePie.php

Lines 1732 to 1734 in adc91d4

	$headers = [
	'Accept' => SimplePie::DEFAULT_HTTP_ACCEPT_HEADER,
	];

should be configurable.

Describe alternatives you've considered

N/A

Additional context

I can submit a PR.

[Art4]

By allowing a PSR-18 HTTP client the headers could be changed easily. See #777 and #826.

[jtojnar]

I am not sure changing headers is a meaningful operation. SimplePie can make multiple HTTP requests and you probably do not want to set a single Accept header for all of them.

Supplying a custom PSR-18 client with middleware that adjusts headers per request will be probably a better idea.
And you do not need to wait for PSR-18 support, you can pass a custom client, that does whatever with headers, today. We do that in selfoss: https://github.com/fossar/selfoss/blob/c49337513662e772c52235f3617f98a13071cdac/src/helpers/FeedReader.php#L33