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.