feat: Implement PSR-18 ClientInterface in IClient

[SMillerDev]

• Resolves: HTTP client docs documentation#2196

Summary

This makes sure the IClient is compliant with PSR-18. This means that any PSR-18 client could be dropped in as a backing client in the future, and it also means that any library using PSR-18 to communicate with a HTTP client can use IClient

TODO

• Make sure all CI works

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[come-nc]

composer run cs:fix should fix the code style.

[provokateurin]

Missince @since 34.0.0

[provokateurin]

Suggested change

	/**
	* Sends a PSR-7 request and returns a PSR-7 response, part of the PSR-18 interface.
	* This is a thin wrapper arround the Guzzle Client implementation
	*
	* @param RequestInterface $request PSR-7 request interface
	*
	* @return ResponseInterface PSR-7 response interface
	*
	* @throws \Psr\Http\Client\ClientExceptionInterface If an error happens while processing the request.
	*/

This is not needed as the phpdoc is also inherited from the interface. Also this usually leads to the docs getting out of sync later on.