feat: add HTTP client foundation and configuration

[algebraic-dev]

This PR adds the basic structure for the HTTP client, including client configuration, possible errors, and small changes to other modules.

• Foundation: feat: add HTTP client foundation and configuration #14546
• Connection: feat: add HTTP client connection #14547
• Agent: feat: add HTTP client #14548

[mathlib-lean-pr-testing]

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[leanprover-bot]

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[TwoFX]

Is there a specific motivation for this feature? Is it inspired by an HTTP client in a different language?

Either way, the semantics seem debatable here. It seems that if we're downgrading anyway, then following the redirect should be fine.

[algebraic-dev]

I think that's good for avoiding repeating the redirect when something has a non-idempotent method, like payments or things like that, where we can't allow the operation to happen twice. Maybe I should change it to a validation function that runs on every redirect and decides whether it should change like in golang.