feat: add option to limit concurrency

[costela]

This adds an option to limit the circuit's concurrency:

circuit := hoglet.NewCircuit(
        func(context.Context, any) (any, error) { return nil, nil },
        hoglet.NewEWMABreaker(10, 0.9),
        hoglet.WithConcurrencyLimit(100, false),
    )

The limit may be non-blocking (fail immediately) or blocking (holding off on calls over limit).

When blocking, context cancellation is respected to avoid requests piling up, but these do not count towards the failure rate. This means the circuit will not open just by having too many calls time out while waiting for a slot.

[obitech]

LGTM