Add `Delegate` as an easy method for extending the delegation chain

[smoyer64]

Given I have a UCAN delegation token with (one of?) my DID(s) in the aud field, add a Delegate method that makes it easier to continue the delegation chain. This can be accomplished but the New constructor but almost every field must then be copied by the library user.

The method signature would be something like:

func (t *Token) Delegate(privKey crypto.PrivKey, aud did.DID, opts ...DelegateOption) (*Token, error)

And DelegateOption would include the following implementations:

• WithAttenuatedCommand
• `WithAttenuatedPolicy
• WithAdditionalPolicyPredicates
• WithoutInheritedPolicy