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.

[SMillerDev]

Anyone know how I can get the cypress tests to pass?

[provokateurin]

Just need to restart them until they pass 🙈

[nickvergessen]

To the main-maintainers: there are 2 things that should to be checked/confirmed when we expose PSR interfaces:

• The package should be required in 3rdparty (currently it's only indirectly pulled in as a sub-depedency): build(deps): Require psr/http-client as it's exposed now 3rdparty#2383
• The package is required in the OCP (ref fix(ocp): Install required psr/http-client that OCP depends on now nextcloud-deps/ocp#37) currently it breaks apps that use it: [master] Update nextcloud/ocp dependency notifications#2916

[github-actions]

Hello there,
Thank you so much for taking the time and effort to create a pull request to our Nextcloud project.

We hope that the review process is going smooth and is helpful for you. We want to ensure your pull request is reviewed to your satisfaction. If you have a moment, our community management team would very much appreciate your feedback on your experience with this PR review process.

Your feedback is valuable to us as we continuously strive to improve our community developer experience. Please take a moment to complete our short survey by clicking on the following link: https://cloud.nextcloud.com/apps/forms/s/i9Ago4EQRZ7TWxjfmeEpPkf6

Thank you for contributing to Nextcloud and we hope to hear from you soon!

(If you believe you should not receive this message, you can add yourself to the blocklist.)