feat: ValidateToken takes options, too

[Peeja]

This makes it practical to use on its own, outside of ValidateInvocation (for validating a delegation, for instance).

Required for fil-forge/sprue#15.

PR Dependency Tree

• PR feat: ValidateToken takes options, too #28 👈

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[frrist]

nit: replace this with ucan.Now()

[frrist]

withConfig is wearing an Option "costume" to do something options aren't supposed to do: replace the whole config rather than tweaking a field. That has a few knock-on costs:

• Order-dependent in a way the type can't express. withConfig only behaves correctly when it's alone; combined with any real option it either clobbers them or gets clobbered, silently.
• Two ways to express the same intent (build from options vs. inject a pre-built struct) for one underlying need.

Can we instead split this, something like:

func ValidateToken(ctx context.Context, tok ucan.Token, options ...Option) error {
    return validateToken(ctx, tok, makeCfg(options...))
}

func validateToken(ctx context.Context, tok ucan.Token, cfg validationConfig) error {
    // existing body
}

Then ValidateInvocation and the proof-chain loop can call validateToken(ctx, …, cfg) directly, and withConfig goes away.

[Peeja]

Ah! That's the way to do it. ✨